Without even addressing her arrogance and expectation unrealistically of your full support, what is your stand on her cheating at all?

I know you're saying she's an adult and can make her own decisions but the question isn't that, it's whether or not as her friend you should support something like this. If you're ok with cheating, then so be it, support her as best you can but i get the feeling deep down even though you say "it's not the problem" I think you do have a problem with cheating. If you do what kind of a statement are you making that it's ok with you?

You have no moral obligation to support someone that you know is bringing her own pain and suffering into her own life by cheating. Tell her that you support her as a friend but not in the adulterous behavior. Anything that has to do with her adultery partner (I refuse to call it a boyfriend) you need to opt out of supporting her in. If she's doing waht she's doing and crying about it, I have no sympathy for someone that sacrifices the feelings of others and their needs (her husband, the partner's wife and kids) and then expects support and sympathy herself, it's a completely selfish position to take.

She is putting you in a difficult situation here and you deserve better. It matters sure, that you've known her for years but all the more that you need to step away from her until she gets her head on straight. Perhaps NOT supporting her will give her the wake up call she needs.

And.. regardless of if there are kids involved or not, if she is with a man that is married and he's not in an OPEN relationship where his wife knows what's going on, it's deceit and 100% cheating, not anything less and I don't care what angle someone looks at it, it's wrong.

You need to move on if only for awhile until her clear path of destruction meets it's inevitable end or she comes to grips with the reality of what she is doing.