I can see the let them talk attitude but that's not my nature. I'd approach all of them and give them the vets advise. Better yet if you can find out who this is coming from and target them, it's usually one person.
If that's not your personality, and I'd guess it isn't then let them go, but they will keep going. Me I prefer to head them off. I stick the vets advise to my stall door with a big red circle and arrow and a note saying b1tch now!
I think bullies need a strong hand, it just depends on your personality and what you think you can do.
That's just going to escalate the gossip and if I was the BO, I would be asking the write of such an inflammatory note to please take their horse elsewhere!
Your horse is injured... are people talking? Sure... people talk about anything AND everything! The entire thing is probably a misunderstanding about someone feeling that in their opinion the horse should be locked up 24x7 whereas you are following your Vet's instructions. If anyone says anything else, just go "Oh she's doing just fine thank you, I'm taking care of her per the instructions from the Vet that treated her" and then turn and occupy yourself.
Furthermore, if it was your BO that confronted you about this, you REALLY need a new place to board. Not only is a BO that can't stand the sight of blood not doing anyone's horse any good but the BO should KNOW what the Vet's care orders are.
My BO isn't present for every Vet visit, she has a life and I certainly don't pay her enough to demand that she watch the Vet treat every lil boo boo or give my horse his annual checkup and go "well... I think we should change X, X and X and see how that goes" BUT my Vet either goes up to the house with me after and we'll all have a chat OR he calls her (if it's something big and she's not home) OR he'll write me up a page of instructions.
So when my horse gashed his foot open and I texted him pics, he had her on the phone before he even got there with instructions on preparing a stall, how he wanted him trailered (he was at my place right then, not the barn) and so on. The BO should never be out of the loop even if the care doesn't require her or her barn hands to do anything different than normal.