I can't believe people are honestly justifying cheating. God, no wonder this world is a mess.
Cheating doesn't make a guy culpable to murder in the eyes of the law. But that doesn't mean he should have done it. Let's not justify a wrong behavior. Even without marriage ,you're not supposed to cheat on your partner.
Also her mother should've advised her to stay clear of a guy so much older than her, especially since she's at a vulnerable age and also had previous history of depression. Age difference is only okay when the younger partner is 25 or above. By that age, a person somewhat understands the world and gets more or less mature.