Is my friend justified in feeling betrayed?

As-salamu alaykum. I have a friend who was in an impermissible relationship with a girl. They agreed they would make things halal next year because of school, and she promised him marriage later on. Everything seemed okay, but one night she suddenly changed and started distancing herself for no clear reason. She ended the relationship saying she wasn’t ready to make it halal and was afraid of sin, but then she started seeing another man about three weeks later. My friend is devastated. He was close with her family and feels deeply hurt and betrayed. Is it reasonable for him to feel this way? He makes dua every day that she feels remorse for how she treated him because he feels disrespected. He’s been in a bad spiral since then and is now afraid to trust or commit to anyone. Are his feelings valid, and is the dua he’s making acceptable - asking that she feel guilt for what she did? Edit: I know many replies point out that the relationship was wrong in the first place and I agree. But please try to see it from his perspective: he was promised marriage next year, and within two weeks she seemed to change her mind and was with someone else. That would hurt anyone. So even if the relationship was impermissible, that didn’t give her the right to mislead him and suddenly treat him that way. She could have ended things honestly instead of giving false promises and then being with another person.