(1) There are two people in a marriage. However laudable our principles, when we intrude into others' marital relations we're asking for trouble. Among other things, it nearly always means taking sides to some extent, and doing so on a lot less information than we like to think we have.
I could stop right there. ;)
Caveat: this is where I go all Uncle Al on the subject. I'm doing so because you asked for all kinds of advice, not because I feel any need to debate these concepts with anyone else, whose mileage may well vary. :)
(2) That said, I'll note this situation as (IMO) a fine example of the superiority of Thelemic over traditional relationships. Two kings who have long since left each other free to pursue their individual Wills would be far better able to handle this problem. Agonizing though it would be for anybody, at least each would have an abiding internal principle to comfort them: the sanctity of the other person's Will, as well as their own, most particularly in marital issues. Without the spiritual dimension, human relationships tend to get blown completely out of proportion, with results ranging from miserable to catastrophic.
(3) Interfering in traditional* relationships tends to get you even more trouble than is already implied in (1) above. I could stop there, too, and probably should. However --
(4) A crucial aspect here is for you to try to minimize the melodrama of an admittedly melodramatic situation, because humans feed in really sick ways on such melodrama: self-worship, self-loathing, and everything in between. The odds are that your friend won't actually want to "lean on you" (as you fear) for advice -- they'll want either sympathetic drama-sharing (I would spare them very little), or help breaking the news to their jilted partner (and/or protection from that partner's hurt and anger), which latter is a non-trivial concern. Folks who pick a time like this to locate their Will elsewhere than their marriage, sometimes get killed for their trouble. Some of what passes for morality is, IMO, a species of insanity, and its belief that one person can own the body or Will of another can turn out really, really, really badly for all concerned.
As a general thing, not knowing these people as you do, I'd probably (if I got involved at all) be simultaneously telling [party A] to do [their] Will in as honest and honorable a way as [A] can find it within [themselves] to do, whilst telling [Party B] that however bad it feels, this is a blessing in disguise, because [B] can do a hell of a lot better than this in the commitment department. If need be I'd also point out that being jailed or dying is a really poor outcome for what would otherwise be a temporary setback, however understandably gi-normous it feels at the moment.
Toxic and dangerous stuff, with multiple ways it can go badly. Hope some of this helps -- I've counseled folks (who were asking for same) in not-dissimilar positions, and you have my sympathies. :(
93 93/93 -- AJ
* AC would have said "slave."