My best friend's boyfriend is having his bar mitzvah. He invited me and I'm going but I have a lot of ?'s

you should ask your best friend. It's her boyfriend. She must know the protocol and if you tell her you lost your RSVP she can let him know and also let him know you are definitely coming. Ask her advice and tell her about the fact that you don't want to spend too much money but ask what kind of gift is acceptable for a bar mitzvah and what is best to wear. She must be familiar with this through him, xx