Derek's amused by it.
If the glares Talia's shooting him mean what he thinks they do, his mother doesn't think it's the least bit funny, and she'll likely tell Stiles all about it before he gets home from his trip.
Isaac: Cora tells me I should ask about dinner with the family last night?
Derek: she would be the one to gossip.
Isaac: so you did go off with some lady?!
Derek: Went out with the family last night and some 40 yr old lady wanted to take me home. My mom was not happy with me
Derek: that's it!
Isaac: was she at least a milf?
Derek: Derek isn't going to going to respond to that if he ever wants to have sex again!
Isaac: I guess someone's busted. let me know when Stiles lets you have your phone back.