To make a long story short: invites went out two weeks ago. People we know are in relationships/married/engaged got to bring a guest. One of our invitees is a middle-aged, divorced friend of FI's mother. She did not receive a +1 because to our knowledge she's not in a relationship. Apparently when she received the invitation without a +1 she called up FMIL and acted rather offended, "You mean I DON'T get to bring a guest!?!?" etc. I gather that she wanted to bring one of her adult children, but I can't be certain.
We got the response card in the mail last week with a decline. FI's mom is upset with us that friend did not receive a +1. However, we showed her the guest list prior to sending out invitations. She saw that the friend was just invited by herself and made no comment or suggestion to give her a +1.
(I personally think FI's mother's attitude is misdirected at us...I think she should be upset with her friend for being a b*tch).
Anyways, were we in the wrong for not giving her a +1?