It's a judgement call.
Your friend's husband is divorcing her for alleged emotional abuse the she has perpetrated on him which he claims he 'can't get over'.
Your friend has admitted to you that there is substance to his allegation as her jealousy has been out of control.
Has this jealousy taken the form of her checking his movements, demanding to know his whereabouts, prying into his personal correspondence, and generally acting the part of a suspicious wife who believes her h may be having an affair but is unable to find proof of his infidelity?
If this is the case, I suspect that the slippery arsehole convinced your poor friend and others that she was an unreasonable, possessive, harriden while he was, indeed, having it off with another woman over a period of years. What a twunt!
As for him being 'unable to get over it', I suspect that this is in part or in whole due to him getting his leg over with yet another woman who, in the fullness of time, he will claim to have met after the divorce is finalised.
Of course, it's probable that revealing your knowledge may end your friendship but she deserves to know that there was nothing wrong with her mental health or her twat radar.
No matter what the repercussions may be for you, exposing this particular secret at this late date is likely to make your friend feel infinitely better about herself - and that's what counts.
The truth always wills out and this twunt's chickens are long overdue for roosting.