At restaurant 4 yr old kid repeatedly kicking seat of friend with back injury - how would you handle?

Yesterday I was at a nicer restaurant with an older friend who had a back injury from a car accident several years ago. A boy approx 4-5 yrs old was sitting in the booth behind ours and repeatedly kicking the seat, hurting my friend. She stuck her head around and in her very sweet way asked the mom to stop him from kicking the seat as it was really hurting her. After about 5 min I saw her visibly in pain and asked her what was going on. She said the kicking had picked up again and was nonstop. I stood up to see over the top of the booth seat. In a polite, friendly manner with a smile I asked mom if she could get him to stop the kicking, as it was really hurting my friend who'd had a back injury. She snapped, "I don't know what you expect me to do! He's sitting here behaving himself." I politely suggested that the two of them swap seats, as there was no one behind hers that would be disturbed by kicking. In a huff she snatched him up and sat him beside her on her side. It solved the problem, and my friend was able to dine without further pain. How would you have handled the situation? I wouldn't say he was "behaving" if he was continually kicking someone's seat.