I’m going to be a bit more sympathetic towards her and what happened.
True, MS is no excuse. There never is an "excuse" for infidelity, and yes kissing is definitely a line that shouldn’t be crossed. Let’s be very clear on both these issues. I am neither minimizing or dismissing what took place.
However… there are some considerations that might explain and possibly mitigate what took place. Not excuse, but maybe help with the next steps.
First and most important is the self-confession. She could have blocked OM, avoided him, refused him entry into the home and given you some excuse or reason and you would never have been the wiser. Coming forth and confessing is an indicator that she acknowledges what she did was not only wrong, but seriously wrong. There is no "it was only a kiss, no harm done" that we so often see in it’s various forms here on SI.
Second: MS isn’t an excuse, but we all can have periods where we question our lives, our validity and our value. We all seek validation in some form or another. Most of us in healthy ways: we feel validated when we can pay our bills and have some cash left over, we feel validated when a customer compliments our work, we feel validated when we get a raise, we feel validated when we look at our family all happy eating dinner.
Others need unhealthier forms of validation. That can be as "innocent" as light flirting, that can be as harmful as needing to realize the flirting isn’t "innocent" or light but full of intent. Note I started by calling this unhealthy validation – it takes lots of forms. Bullying is validating superiority, wining an argument can validate superiority…
I can fully understand that something like a damning illness can shatter self-worth and validation. Don’t know the extend of her illness, but things like limited work or no financial contribution, being told bad news again and again, limited progress… none of this adds to self-worth unless it’s already at a very good place originally.
Excuse? No. Reason? Possibly.
Third: There are people – both men and women, although I have mainly encountered this with men – that hit on anything and everything they can hit on.
Over the years I have witnessed and worked with a couple of guys that seldom slept alone, and often slept with someone other than their partner. These men generally fronted confidence (often with little content), assurance, they mastered contact, mastered reeling you in and mastered making you bite. All attributes a good salesman could put to good use. Only – if misused they are selling their bodies at the cost of yours. We can call them predators, but frankly they only catch what bites their lure.
Fishing reference? I have a friend who generally catches more fish than I do. The reason being he cast further and more often. Covers more ground. It’s the same with these guys that they get more women because they are constantly casting and covering a lot of space. Had your wife’s friend been there when he was in your house he would have hit on both of them.
Now – doesn’t excuse her actions but might explain why she was reeled in.
How to move on?
Well… IMHO you have two good options. Neither easy, but both "good" if they fit what you want.
You can divorce.
You can reconcile.
You don’t necessarily need to decide which option you take now, but eventually you reach that fork.
For NOW this is what I would do:
Make your wife realize you appreciate her telling you what happened. That her honesty goes some way in helping you deal with this.
Ask her how she can assure you it’s the truth – the number of events, what happened and so on.
Make it very clear to her that NOW is the time for truth. Complete truth. If you trust yourself to it then make her a promise: No matter what she shares NOW will not automatically lead you to divorce – give her an "amnesty" for 30 days. (It’s to your advantage anyways because it’s never smart to decide in anger or pressure). If she admits they met 4 times and/or had wild sex on the couch… NOW is the time to learn about it.
Honestly – learning about more now will cause less damage than learning something trivial in six months. Learning it then indicates your wife hasn’t trusted you for the truth, and lack of trust is a marriage-killer.
There are certain actions that experience tells us will help kill affairs.
Key amongst them is making all stakeholders painfully aware of what’s going on.
Is the OM married? His wife should be told. This isn’t for ethical reasons or because it’s the correct thing to do. It’s because for HIM your wife is just the catch of the day, and if the catch of the day bites back he throws it back into the pond. Rejection is a great killer of romance.
I’m guessing his wife knows he’s a player and this is the umpth time this happens.
His boss should know. He is a salesman? Let his boss know that he is a predator and took advantage of your wife. Even if the boss claims it’s consensual you point out that this was on business time and not a service you asked for… Make it totally clear that you are willing to notify the better business bureau or whatever and that any future business is 100% dependent on how the boss reacts.
Other than that… You two need some time to decide your next steps. This wont heal or go away in a week or two. This is something you will be dealing with for months. The key is getting the truth. You can rebuild from a firm foundation and that foundation tends to be a combination of past behavior and truth.