He has a big problem anyway. The thing is he is with a girl that would like to meet with her ex bf, she talks to him, texts him etc.
In conclusion, she's not over him, and if she is not, In10se is just her second option.
I think there is not much he can do, its pretty much doomed. He can:
a) confront her
b) go kind of ghost
c) break up with her
In either a) or b) there is a small chance that this girl will realise In10Se is the best option and will stop all contact with the ex, but this is in my opinion a small chance because she's already talking to her ex bf, wanting to meet him, and creating all this big problem for In10Se.
If you're with a girl that has High Interest in you, she's not going to bother you or mess with you by talking to an ex, or by doing stuff that may jeopardize your relationship. She may do anoying stuff to test you, ie: saying she may meet her ex bf for lunch, but she's actually faking it, she has no intention of doing that, she's just testing you, so here is where you set a firm boundary.
But if she is doing this stuff because she ACTUALLY WANTS TO do it, she's not the right woman for you; she just wants to see if she likes her ex bf more than she likes you, which says she is not into you as much as you're into her, which is bad.
In10Se, I would contact her less, go have fun, meet some girls, and when she asks why you're absent, just tell her in a calmed way that you do not accept that behaviour, that if she thinks it is acceptable to talk to other guys and specially an ex, you cannot continue to have the same relationship with her, so you turn her into FWB (without telling her of course), and then she can decide what she wants.