I think you’ve been put in a very difficult position op.
I still think it would have been kinder to leave you blissfully unaware of your exh’s misdemeanours during your own marriage. That information is not helpful to you.
Perhaps she felt it her “duty” to tell you? I think she’s misjudged that and not considered your feelings.
Clearly his wife wants someone on side or perhaps supporting her through her marital issues. Is the mother of her husband’s children the right person to dump all this on? I don’t think so.
Fair enough if you’re close enough friends- she could have confided in you about her husband’s infidelity without it involving your marriage too?
Its undoubtedly affected the amicable parenting relationship you have with your ex so opened a can of worms for you and potentially your dc.
However, it’s done- can open, worms everywhere!
You sound very sympathetic and want to console your friend but think carefully about how much you can give of yourself to support your ex’s wife through a marital breakdown and whether you are too emotionally involved / linked to your ex to do this?
I would encourage her to confide in friends and family so that the onus of “support network” isn’t placed entirely on your shoulders.
You are trying to process this news yourself so don’t feel guilty for stepping back and employing some self preservation here x