I can't imagine Lisa turning on Vic because he dated someone younger than him. Back in '19/'20 (I think) Vic was also dating a girl who's significantly younger than him, she was in her twenties. Yet, back then, Lisa was still very vocal about supporting him.
So, I don't think this has anything to do with him dating younger women.
I believe Vic's just not good when it comes to being faithful in monogamous relationships. Vic starts dating Rachel (his ex), Lisa befriends Rachel, Vic cheats on Rachel, maybe multiple times, Rachel breaks up with Vic, Lisa ends her friendship with Vic because she wants to support Rachel.
But that's all speculation.