I don't think the age gap is particularly alarming, I definitely would have been interested in a 50 yo at 31, always found men my old age unattractive and boring but the fact that he is so openly doing this is vile.
I totally blame him, he is married and has a responsibility to his family. She may well know exacty what she is doing also but it is also possible that he has been bullshitting her.
I had a BF when I was about 19, he was late 20s, he used to stay at my house a few of times a week, basically all weekend and often during the week too. He said he had a couple of kids which he visited regularly. Turns out he was actually living with them still and was still supposedly in a relationship with her.
I wasn't a naive 19 yo, I did feel there was something a bit off but the fact he stayed over and messaged me/called me every day during the day and evenings meant I didn't think it was that he was actually still living there.
I digress, and I am not letting her off the hook, but it really irritates me that some pp actually are putting this onto her.