I'm curious what would happen if you returned a warm gradual smile at her non-emotional look at you. Probably add a wink next to it. Or walk up when she smiles back (who wouldn't at a sexy man like you) and say hi.
Hey, how have you been?
The funny part is she's with her friend, and she's likely not going to say "yo ex-lover". She'll likely tell her friend, "This is Sam, he's my friend". So you two have this friend dynamic on the outside with a secret flirting inside feel that her friend won't know about. Now that's hot.