I'm so sorry for your loss of your dear bun
My Moose died at 1 from unknown issues... but as in your case it was likely something very serious and possibly genetic.
It's very good you left his body with her for a while. Usually when that is done, the surviving rabbit will grieve, but they will understand what happened and won't slip into deep depression.
Is she eating and pooping OK? It's normal for buns to be a bit more quiet or behave a bit differently while grieving, but she should eat and poop her normal amount. The best thing you can do for her now is to try to spend as much time as possible with her and monitor her health. You might consider getting her a stuffy to snuggle and groom (but supervise to make sure she doesn't chew it). If she does seem to be going downhill, often getting another rabbit (even if just a foster, to be a neighbor for a while) will help a ton. I read one account of a grieving bun that would only eat when another bunny was around, so a friend would bring their bunny over daily so that bunny would eat.
Again I'm so so sorry... give her extra snuggles, it will help you both with this process.
((((Binky free little bunny boy))))