How do I handle this situation?

My SIL has planned a group camping trip with all our friends. My best friend S was invited because she’s become friends with everyone.

SIL has done all the legwork of planning and booking the sites. After booking S asked if her boyfriend could come. S has been dating him over a year but due to covid, no one but me has met him. He’s nice but about 10 or more years older than us(over 40) I’ve met him 3-4 times.

SIL comes to me and my fiancé basically saying she doesn’t want S’s boyfriend there and doesn’t know how to tell her. She feels like since it’ll be the first time in over a year everyone will be together that it’s not the time for everyone to meet him and also says she’s not comfortable being around him not knowing him personally or where he’s been. She ultimately decides that she’s not going to invite him and makes up a lie basically saying the campsites are full and S should have said something earlier. S then tells SIL “awww damn, I totally assumed he was able to come, let me know if someone cancels”

S then texts me and basically is like “I’m super bummed BF can’t come”

How do I respond? I understand both sides because we use this yearly trip to let loose and just get drunk but I also feel terrible that S was lied to. It’s my SIL though so I don’t want to cause drama.