Waving at your old friend when your new crowd is watching is a small test of loyalty. Pass it.

Loyalty is only tested where it costs something, and a new crowd watching is exactly that kind of cost. Pretending not to see someone protects a standing that was never worth much and teaches you something about yourself that is hard to unlearn. The person you did not wave at will notice, and so will you.

Say hello properly, with your name and theirs, in front of whoever is there. Introduce them if it fits. It takes four seconds and it settles a question about what kind of friend you are.