What's the issue? She's your girlfriends mother, you two simply need to not be at war with each other, that's all. That she doesn't want to have dinner with you is a good thing, not a FML.

She goes there with her lover.

If she was just tagged and not the one to actually check in it probably wasn't her idea to go there. Either way people are allowed to change their minds about resturants. The only FML here is the fact that you let something so trivial get to you this much.

It could be that she is humoring her mother. Depending on how old she is, like if she is a teenager, her mother may have just decided that's where they are going to dinner. In which case she would have had to just suck it up. If she is older, well I can admit that I would go to a restaurant with my mother that I wouldn't go to with my spouse. The reason for that being, in the long run, I will likely have less time with my mother unless my spouse and I break up and would like to see her happy and therefore am willing to humor things even if they make me flinch as long as they aren't things I fully can't stand. When it comes to my spouse, I want us to go to places we both can enjoy together. I don't like seafood so we don't go to seafood places together. He doesn't like coffee so we don't go to coffee shops together. But we both like hispanic food so we go to hispanic restaurants together.

To me it seems as if your girlfriend's mother doesn't care for your input as much as she should. If that's not what the FML was implying then I'm at a loss for what it could be about.