He's an ex. You don't handle this, you stop all contact with him, block him everywhere and stop caring what he does with himself now. You are no longer together and therefore, as sad as it is to say, he can do whatever he wants.

You need to get over this, cut him off and not allow his toxicity to impact your healing.

Yah no I would be dropping him over this. There's no rebuilding trust because he lied to you and hid it from you.

If he had stopped at the airport and getting her number for educational reasons, fine whatever. I would have been mad and jealous but moved on.

This isn't just borderline cheating, I would consider this cheating considering the flirting and offering to take her places that he knows are romantic or you found special.