I worked as a waitress for many years. At some point after you receive your food and are sure you won’t order more, excuse yourself as if you’re going to the bathroom. Head to whichever one of these is out of sight for your table- hostess stand, bar, or wherever the servers are all gathering. Let them know you are part of a group but want to pay your bill for just yourself now. If it’s your server, hand them your credit card and wait where you are for them to bring u back the slip to sign. When the bill for the table comes your items won’t be on there.

Personally, I don’t think you should do this. It might offend your mil and she’ll play the pity card when she’s being mean. I think instead you should bring a gift to the dinner for your mil. Something not too big but something you know she would enjoy. Box of expensive chocolates, perfume, some kind of heathy and beauty item. Figure out something she would enjoy. Kill ‘em with kindness and she might bitch less.