Guest bringing someone other than name on invite

I send out invites early because the room blocks expire really early for some reason and I invited a friend from out of state. Within days of my mailing the invite she told me that she and her husband are getting a divorce. Once she got the invite she called and asked if she could bring a friend so she didn't have to travel alone. So since we had already saved 2 seats for her it wasn't a big deal.

ETA- she also offered to pay for the mystery guests plate. I told her absolutely not.

I don't think it's rude...if you addressed it to "Friend and guest", would you expect her to call ahead and have you pre-approve her date? You invite a couple to be courteous to the couple, but if you give them a plus one (husband or not) it's their choice.

It's rude on her part, but I wouldn't fuss over it.

Why does it matter when she is still filling the seat u have her? Maybe her husband can't go she isn't required to call and tell u

I think it's rude that she didn't give you a quick text to check, but I don't think this is a big deal at all in the grand scheme of things. You budgeted for 2 and you know the mom, so who cares? I personally don't understand why some people act like this is a really egregious offense, but that's just me. One of my co-workers brought a friend instead of her husband to my wedding, and I didn't even think it was a big deal until I came on WW and saw that apparently it is. Oh well.

Definitely an etiquette fail on her part. Celia is right that invited guests are unique individuals, not just fungible entries on a list. But I also think this is one of those times when human beings matter more than etiquette rules. You love this friend and you want her to enjoy your wedding, so let her bring her mom.