This makes my blood run cold, purely from the point of view of the child's safety. Aside from the risk of falling from a table and injuring themselves, climbing on a table during a dinner with hot dishes, sharp cutlery and fragile glass items adds a whole new level of potential injuries. I bet SIL is not within arm's reach every time he is up on the table, is she?
Then add that it is dreadfully rude to allow your child to put his feet where other people are about to eat. How revolting. And to 'un-set' the table to allow him to do so? Argh!
At some stage your mother needs to decide whether she is willing to be held hostage to increasingly bratty and inconsiderate behaviour. Imagine this child's table manners as a teenager if he is allowed to continue to do as he pleases uncorrected. 2 is old enough to learn to differentiate. I would physically take him off the table and say "No, we don't climb on the furniture at Grandma's". If his mother kicks up a stink I would probably only mention the safety aspect "Look, I'm sorry, but it makes my blood run cold every time he climbs up, I'm so afraid he will hurt himself, and I'd never forgive myself if I let that happen. Isn't he precious? Did I tell you about the time he was visiting and got into the beandip?" Then repeat, repeat, repeat.