It could be a totally platonic relationship between them. If she’s been hitting on him for awhile now and he’s just now responding you may want to ask them what’s up. Still, you don’t hit on someone that’s already married. Whatever is going on, you need to put a stop to it. Make sure they are never alone together.

You're missing the biggest problem here. He lied about it and she only found out later

Alcohol wasnt the only thing in her mouth that she swallowed that night.

Damn, sounds like your husband was hitting that this time

Looks like she’s hitting a home run, while you’re getting no-hit.

Time to bring in the long relief.

Sorry for ya OP time for a couples therapy or a lawyer.

First she's topless in the pool with him and now drinks at her house?

The fact that he lied is a red flag

Instant divorce

This is wonderful testimony to the fact that no one should ever give up on her dreams. If you just keep working and trying, you will succeed!

Offer to do the same with his best friend, her ex fiance, to get back at them