My S went on my morning walk with me today. I told him that I have no problem at all with him having dinner with his mom tonight. We talked a little about things between his mom and I, but then talked about other stuff too. It was nice.
This is good.
As for what to do when she gets there, my favorite thing is for you to be BUSY. Be in the middle of home improvement project or something in the yard, or be dressed ready to go for a jog or something. Then say something very polite, like your "have a good time!" line, and then go do what you gotta do.
You could also be dressed in a snappy new shirt, smelling of new cologne, do the same "have a good time!" thing, and then say "Sorry to be so short, but I'm already late," and then -- booom! -- you're outta there.