If the child is accustomed to her, what is the big deal? Other than you'd be uncomfortable if she's there? And I'm not sure what you mean by 'too many people'...you, father, SM and hospital staff (usually one nurse at a time and maybe the surgeon stopping by afterward) doesn't sound like a lot. Trust me, if the staff thinks there are too many people, they will ask someone to leave. But...they will also request people to leave if they feel it is causing stress to the child. Which includes any adversity that may be going on.
When DS had his eye surgery, his father came from CA (1800 miles away). Besides medical staff going in and out, there were at least 4 people (myself, father, DH
, and my mother) in DS's outpatient room. Luckily the room was big enough to accomodate us all and we all got along for DS's sake. Same with SS
when he had his tonsils out, which was even more....'tense'. That was DH, myself, BM
, her DH AND SS's half-brother. But regardless of what the adults
thought of each other, we ignored that all for the sake of SS. You would never know that there was a tremendous amount of animosity especially between DH and BM.
I really can't see how you can prevent her from being there without creating a scene. Is that what you want? Wouldn't it be simpler to be the bigger person, suck it up for a few hours, and ignore any attempts by the SM to antagonize you (if that's what you're worried about)? Think about the stress it would cause YOUR SON if you play into SM's hand and respond to her negatively. As the saying goes, it takes two to tango and if SM doesn't have a willing audience for her barbs, it takes the sting out of them and your son will be much less stressed about the situation. It will be scary enough for him to deal with what is going on, don't compound it by engaging with the SM and lowering yourself to her level.