Should I delete my cousin's boyfriend off facebook?

[Rita]

Basically, my cousin is going out with this guy and he seems to like her and she likes him back. He added me on facebook and he has chatted with me a couple of times. But tonight he was a bit flirty like " oh delete your profile pic cuz it makes u look older" blah blah, im scared that he is trying to make me and cousin to fall out. i dont know what to do? Like i dont wanna be rude to him but at the same time i dont want to talk to him. what if he copies the messages and sends it to my cousin?
should i tell my cuz or will she be upset? what if she finds out i have been talking to her bf?
I dont flirt with him or anything i just act cool
She knows that I have him on facebook