MIL inviting friend to bachelorette

UPDATE: thank you for all of the feedback. I wanted to give an update and clarify a few things that seemed to create confusion with a few commenters

1. It is MY mother I am talking about. I referenced her as MIL in the title because I am posting on behalf of my fiancé (the bride.)

2. Yes, I realize that this is a "nothing burger" in the wedding drama universe. I felt it was just interesting enough to merit some discussion on this forum. I thought I made that clear in the original post, but I caught quite a bit of "WTF?" in some of the comments.

3. "Moms don't come to bachelorette parties:".In my family, it is common for moms to attend. Mostly because we usually just have one or two bridesmaids, and that would make for a tiny bachelorette. Also, we have great moms who make the party fun. It is not common in the bride's family, and it lead to some confusion on their part.

The Update: We are letting it slide and mom is bringing her friend. Should she have asked? Probably not. Is it going to ruin the party? Definitely not. I am truly blessed with my future wife and her family, and I appreciate their flexibility during the wedding planning. I am also blessed with a loving and enthusiastic mother, who let her personal desire override her common sense this one time. No one is perfect, and we are extending her grace.

I am also thankful for all of you on Reddit that provide me with a forum to pose these types of questions. The wisdom of the crowd is often a very real thing. Peace!

------Original Post-------

I (57m) am posting for my fiancé (52f.) Not a huge drama, but future SIL wrote my fiancé to tell her that my mother asked if she could invite her friend to the bachelorette party. I'm shocked, because she usually has great judgement when it comes to social etiquette. My fiancé only met this friend briefly, and we aren't sure what her reasoning is for doing this. My sister is coming, and she has met most of the other attendees, so It's not like she will be among strangers. The event can definitely accommodate additional people, it just feels weird and inappropriate.

I offered to do whatever my finance wants, everything from calling mom and asking WTF, to butting out entirely. She is not sure of a graceful way out of this, and is leaning towards just saying yes to avoid bad feelings.

Anyone have anything like this happen before? Honestly, if this is the only wedding drama, I will be thrilled.