I hope the conference went well.
Your company sounds like it needs to establish a no fraternization policy. Many companies just have it with a supervisor and underling policy. If they adapted that, it would be the end of her.
I agree with your stance on not wanting to go to this conference with her.
As to her messing with a client, now she's in dangerous territory. When it goes bad, it's going to hurt the bottom line.
Most states (all of them may be actually) are at-will employers. Which means, technically they can fire you whenever they want as long as they have just cause. If one of the wives or someone took her to HR for sexual harassment, she could be fired. Her HR file should have documentation on her behavior and the company law department should be working on something to get rid of her. Honestly, if they really wanted her gone, they would find a way. Announce a slight restructure and eliminate her position, look at every detail of her expense statements, look for abuse of electronic communications, use of a work cell phone to make (excessive) personal calls, etc.