I can’t offer any other advice than what I would be offering your son in law.
Well... except for this: Maybe he would be well served if you pointed him to this site. To-date there isn’t anything you have shared that can in any way harm him or your daughter or their future paths – be it divorce or reconciliation. If you do this then be prepared for the inevitable posters that will call your daughter every conceivable foul term imaginable, and just focus on the already mentioned phrase: love the sinner and hate the sin. Some posters tend to have a hard time defining between the two.
You can support your daughter and guide as much as she allows without approving or supporting her decisions that are wrong – be it clearly wrong or wrong from your POV.
Keep in mind that a correct decision might not be one you agree with. Like she can decide to divorce her husband and that is one of the two possible "correct" decisions when dealing with infidelity, even if YOU think she should work on the marriage.
If I were in your shoes, I would encourage your son in law to have the following conversation with your daughter – HIS wife. It’s more of a dialogue but here goes:
"Wife. I love you and have always envisioned a life with you. I know we can improve and grow, and I would be willing to do that with you. However, I have realized one major thing - A key factor: I do not want to share you. I do NOT share my wife any more than I would expect you to accept me sharing my love and fidelity with someone else. I do not condone infidelity.
While you are choosing to be in infidelity, I am at best sharing you. I have already lost you and of two evils accepting that I have lost you is better than sharing you.
I therefore absolve you of any moral and ethical expectation there might be between me and you as husband and wife. You are free to be with OM, date OM, stay over with OM or whatever. But not as my wife.
I ask as a sign of decency and respect that you keep OM away from the home and are discreet about him around me, but that is totally up to you.
There is a process in place for how marriages are legally terminated. That process is fair, and I have all intentions on following that process as best advised by legal counsel. There is no rush – I’m not trying to get this over with ASAP, but I will start preparing and following through with the formal conclusion of our marriage.
It’s inevitable that there is pain, and I’m not really going to try to be your friend. But we can be good coparents and eventually have an amicable relationship.
If this isn’t what you want, then you have some time and space to let me know. But it must be very clear. If you tell me, you want this marriage and are willing to commit to accountable NC with OM then we could start the process of reconciling, but until and unless I get that clear message, I’m just assuming you have committed to infidelity. I’m not forcing you to do anything. It’s totally up to you, but I do know what I am going to do."
And then he walks away. Makes a sandwich or does some laundry. If his wife comes along and starts
"I cheated because you don’t talk to me" or "You never take me out" or "we are always broke" or "you have bad breath" or whatever good reason she has for her decision… his standard reply to (nearly) everything is:
"I am sorry you feel that way. If we were working on saving our marriage, then this is something we need to address. However, since you are committed to your affair there isn’t any gain for either of us to go down that path."
She asks him who get’s the house and who gets the Ford:
"I am too attached to this marriage to sensibly talk about the process of divorce. I will seek legal guidance and follow that advice. Want a sandwich?"
She offers him intimacy:
"aren’t you cheating on your boyfriend? I thought I made it clear I don’t condone or participate in infidelity."
She asks to do family activities:
"I don’t think it’s sensible to behave and act like a family when the reality is we are divorcing"
Basically, he detaches from confrontation. He simply accepts the facts as they are. Within a reasonable time if she doesn’t commit to R, he contacts an attorney and/or research how divorce is best attained.
I know this sounds harsh and tough but if you think it through logically: No matter what amount of pressure or screaming is applied if your daughter/his wife want’s out she will get out. This is a key-factor: your daughter can not be forced to remain in this marriage. What the above actions do is clarify what SHE wants based on what she’s offered. Your SIL is refusing the situation where his wife can have an affair and be unhappily married at the same time. He’s offering her the options of trying to create a happy marriage by forfeiting the affair OR the freedom to do whatever she wants short of remaining married.
I would also suggest that within a day or two of this monologue he stops hiding the infidelity. If friends, ask them over for dinner he should be clear: "Would you prefer I or WW come? We are in a tough place right now due to her decision to remain in an affair with (place OM name here) ".
When he exposes he does so as a request for help: "My wife is having an affair with OM. As you can imagine this creates an unsustainable strain on our marriage that can only end with her terminating the affair or us divorcing. I would appreciate if you could impact her to do the right thing because neither of us really deserve to be where we are."
Some will argue that exposure might make reconciliation harder, but IMHO harder is not impossible. What makes reconciliation impossible is infidelity. Exposure is the best tool to kill infidelity.
I know the above sounds tough, and, in all truth, it’s totally intended to create the conditions most likely to lead to reconciliation. It forces your daughter’s hand, while allowing your SIL a path out of infidelity. He can control his pace and it can prevent the arguments that the WS so desperately needs to justify the ongoing affair. It creates all the pressure infidelity, and its consequences should create on the WS to decide to do right. Unfortunately for your SIL that "right" can be to decide to end the marriage. If that’s the case, then IMHO maybe there wasn’t much he could have done anyways.