So, it's probably the second time he's heard that story.

Even if they weren’t best friends, why would she tell that story to your current boyfriend??

WHOA! Guy code...

Why the fuck are you dating someone who was your exes best friend in the first place?

Totally inappropriate of her. Obviously you need to set some boundaries or just avoid her.

Obviously your mom doesn't respect red lines, telling your boyfriend about you having sex with someone else - just like her daughter, now ducking the best friend of her ex-boyfriend. Or her current boyfriend, dating his best friend's ex...

That's so weird that your mother feels like this is appropriate.

Well... were you cheating on the ex with the current? Or are you still having sex with the ex while dating him and she felt it necessary to point out you are cheating?