1. lw can invite whoever they want. 2. sounds like only two people were invited, mom and mil.

So friend is out of line. It'd maybe be a little different if dress shopping was a group activity w/ other friends & activities were aggressively gendered. Though I think more on the level of, it's ok to whine a little in private or say, 'hey, if there's space, I'd be interested to take part' vs whatever is going on here.