Sorry but if B was me or DH I'd expect A to be having a word if we'd left them to just get on with it.
I would hope that B would bring the other one a drink if I/he were at home and making ourselves a drink whilst the other one was incapacitated.
Surely B has a drink and makes lunch - or don't they drink or eat from 9am to close of business? If so, what's hard about making another cup or sandwich? DH and I do this every day on the days we're both WFH. It's common curtesy even if you've not snapped a bone.
Or, if B doesn't stop for a drink, a wee or a sandwich (which is very weird) all day, then leave a jug of water, flask of coffee and a sandwich by A's side before 9am.
You don't have to be at someone's beck and call but if you're in the house, making something for yourself, surely you'd check if the other person needs something. Ill or not. Just poking your head around the door to say 'I'm making a cheese sandwich, would you like one and do you need a drink?' if you're making for yourself. Even our 14 year old knows you ask if anyone would like a drink he's getting a glass of something from the fridge. It's called manners.
And, yes, I would be able to hobble to do all this if DH wasn't home (or vice versa) but if DH is in the house and making his own drinks/lunch, he'd automatically get me something just to make sure I'm not likely to fall (and cause him more work).
Anything else is just odd I'm afraid.