If my son rings up complaining about his live-in partner, which has happened once, when they nearly broke up, I would ask if he wants to hear my opinion, or not, AFTER I have offered what practical help I can.
If by getting involved, you mean offering unsolicited advice or making unkind remarks either about in-laws, or our own adult children, then I agree with all of you who say "keep out of it".
BUT, but, but he is my son, so I am involved. It can never be a matter of indifference to me that he is unhappy.
I will willingly listen to anyone else too who is in the middle of divorce, or the kind of situation that leads to it, when I am asked to listen, and if asked for advice, I will try to give it.