She's sleeping with a married man

so recently my best friend of 20 years has started sleeping with a co-worker that is a married man he is around 10 years older than him and he's told her that his marriage is really not there but they're together because of the kids. Honestly I don't believe it I call b******* but it is what it is she's a bigger girl and I think she really likes the attention and she seems to be super over and love with him you know she bought him $100 beats for Valentine's day and buy some breakfast every day and I made it hotels or gas stations while he's dropping loads. She didn't want to tell me because she knew that I would make comments and don't approve. I found his social media and his wife and her Pinterest and found out that a couple weeks ago she was getting stuff for his health as the board was called my darling husband so it really doesn't make sense that their marriage is not there I feel really bad for the lady because she checks into church all the time on Facebook.how exactly do I continue my friendship with her being there for her but with this feeling inside of guilt because as a married woman I don't think that's right at all.I've made my share of mistakes when I was younger but we're in our thirties she should know better but she says she's going to do whatever she wants to do and she's happy I don't want her to get her ass whooped or get her heart broken .advise welcome please. She's my God sister and best friend. I change the subject because I cringe when she brings him up. She lost her mom last year and went thru major depression and her other best friend who is engaged, sort of encourages them

Edit I'm not doubting her because she's bigger I just know it's giving get a confidence boost and she like I'm going to do what makes me happy type thing.

I am minding my business because she knows exactly how I feel. He didn't see her on Valentine's time day and I'm like what did you expect.

They have a pregnancy scare and he said he would want her to take care of it.

I'm just like really.... Do you not see the big picture here.

Even my husband was like WTH

I just feel like whatever is in the dark will eventually come to light

His social media page has him and his wife in the picture 🤦🤦 ugh I had to vent