Well, as someone who has experienced this, I know how you feel, but like everyone else, this is Sonny's problem not your Mum's! (Good wording Dumas!)
My Mum is incredibly fond of my pony Bailey... she's spoilt her rotten from day one... she was my first pony, and she's been a part of the family for 6yrs now... my Mum hardly comes out but when she does, she'll be out in my paddock patting that darn horse until she falls asleep in her arms. I've told Mum to not spoil her so much, but I certainly don't ever tell her to be quiet, not move etc whilst she's being ridden (not saying that's what you expect your Mum not to do either)... perfect example for this... me and Mum were mucking out my paddock the other day together as I've just come off crutches... Bailey was being ridden in the paddock by her rider and Millie was struggling to get her attention to stay focussed on her... my Mum started going "Ohhh show us your awesome strut Bailey... ohhh look at the pretty pony trotting... ohhhh look at the ponz! Aren't you just a good little girl?!" If anything it annoyed me than Millie! I was rolling my eyes at the stuff coming out of her mouth whilst smiling and shaking my head... but Millie didn't let it affect her... Bailey tried to use us as an excuse, and tried to veer to us a couple of times, but Millie told her no and told her to get on with her work... Bailey is incredibly cunning, and will use ANYTHING to her advantage... distractions are her best friend, but also a great tool for the rider... something to work through, and achieve at the end of the ride. Millie stuck it out, had her walking, trotting and cantering despite the distractions without Bailey using us as an excuse and then got off... we believe in the reward system... do what you wish to achieve, get it, then end on a good note with praise... Bailey being the attention seeker that she is, enjoys being rewarded, and has become SO much more accepting and happy in her work by doing it this way.
♥ Seoul Searchin' for the Lovebug ♥