I'm lumping my flat-chested friend in with the guys as a joke
What makes you think that is a good joke?
I just feel so sorry for her when we go on holiday
You shouldn't be making fun of your friends, especially not for their looks or for anything else that they have no control over. You might think it's funny, but it will only hurt your friend's feelings.
It's just a joke.
It may be a joke to you but it might be hurting the other person's feelings.