The assumption is wrong (for obvious reasons), the ex-girlfriend is still alive thanks to MC being around with his 2nd disciple in the dungeon. The ex gets scarred from the attack and the NTR-man gets fed up with her and finally divorces her. (since she sucks at cooking and spends money to look pretty, which is his own fault because he told her to spend as much as she wants) She goes back to her parents, trying to get past her trauma and better herself. Her old friends come over (who didn't contact her after the ex ignored their warnings), consoling her and telling of their own mistakes and experiences as adventurers. Karen (1st disciple) comes to send a message from MC for plastic surgery to fix her scar. The ex goes on to become an adventurer with her old friends, an finally learns the truth about MC from them. BTW she pays back the plastic surgery from the MC, but MC doesn't want accept it (but kinda does in the end due to insistence).
I'll be honest, I pity her--not to say she's forgiven for two-timing, but the events after leaving MC is pretty sad. She was fired from her job, since she got hired through connections of MC. Her husband cheated on her during the relationship, plus he wasn't telling her the truth and gaslighting her. She tried getting a job at other places, but they wouldn't accept her cause her ex-boss said she won't work in any of her allies establishments, leading her to become an adventurer.
She's been babied by the MC so much that she can't survive on her own to become independent. And when she tries to be independent, she's stopped by others due to past actions. The reason why she two-timed in the first place was due to insecurity with the MC not communicationing to her as much after getting a job (which NTR-man took advantage of). She was destined an unhealthy relationship no matter the choice she made.
The MC and her after: They meet again at a school reunion (have an awkward chat), and even defends him when people speak down to him. Although, it doesn't seem she wants to get back with him due to guilt, despite signs of still having feelings for him. So she tries to get the MC together with Karen and another girl (don't remember the name, this is almost at ch 400 btw) The MC still cares about the ex, since she's technically not only his first girlfriend, but his first friend. But isn't willing to forgive her for what she did.
Btw magic stones improve INT, so adventurers are more wise and smart.