When I was 22 I had an affair with a married man who was then 37. It lasted until I was 30.
I know this isn't quite the example you are asked for, but instructive none the less.
I was seriously in love and worshiped him. It was a power imbalance relationship (as these age gap/work/affair) relationships typically are.
It wasn't real consensual love ( a point many people miss in this situation - because it's easy to say "both adults/ both know what they are doing). It was an older powerful man seducing a younger woman who was already primed to worship him because of the power imbalance.
He never said he would leave his wife. He always was honest and said he had no intention of leaving. He constantly said "you can love two people at the same time" and raved about how much he cared and loved me. He really didn't.
He is still "happily married". His wife has no idea that we had such a long running (and intense professed love) affair. She is perfectly happy and thinks she has a perfect husband.
He was honest of a sort with me so I was never stupid enough to believe he would leave. Married men never do leave. It's only when they get discovered and their wife decides to chuck them out (which few do actually, they soldier on to preserve their family unit, social circle and social and financial status - delete as applicable).
She is really wasting her time.
The easiest way to flush this out is to tell his wife. She'll be rightly furious, throw some plates, put him in the spare room for a bit; but it will shake itself out and he will have nothing to do with your friend again. The sooner this happens the better; don't let her waste years of her life on this man.