Sorry about the loss of your bun <3

Those dwarfs can be so feisty! I think they are compensating for their small size! I do love their spunk though

(also that title is hilarious )

In this situation, I would try to intervene before she nips, and start petting them both immediately when she goes over to him. Apparently he isn’t grooming her fast enough for her liking, so I think petting will help. If you pet them both and swap scents, this might even trigger Toby to groom her on his own.

Also, how neutral is your kitchen? Even though Toby is being very chill, if it smells like him Boo might be getting defensive as a precaution.

Other things to think of, if you haven’t already, are pre-bonding cage swaps, which can really help when one rabbit just seems agro for no reason, and stress sessions.

I agree with Dana – I trying to prevent the nipping is a good idea and giving her what she may want through petting, (grooming) right away may calm her down. 

Based on her past she may need some calming reinforcement when she interacts.   (Wear gloves when you pet them both so that when your glove has Toby’s scent, you won’t get hurt if she nips at that scent aka:your hand…and that way you can react calmly to a nip).  When she nips, you can give out a high pitched squeal as that is saying in rabbit language “Ouch!”   Now she may mean to do that, but sometimes rabbits are biting harder than intended.  I know with Jack he would do this “rough” groom on Rucy.  Bites her face and I would squeal for her, (because she took it too), and after several “squeals”, he began to learn to groom less rough.  So while I know this is a little different in your case, it still might work.   

You mentioned that she was housed with another rabbit at the rescue.  What were the circumstances around that (a group of rabbits, one other rabbit? Just wondering why she was adopted out separately — unless she was part of just a large group?)    Understanding that may help us understand too.  For example: If she was part of a group of rabbits, then she may have developed a way to not be beat up again, but being aggressive….or if she was separated from a bonded mate (passed away or adopted out), then how recent was that? 

That may help give us some insight so we can figure out what might be really going on behind her aggression and find solutions based on this situation.