Did your Vet find out what the underlying problem is that caused Tonsillitis? That would be my big concern as it can be caused from infections of the mouth, sinuses or respiratory tract. It can also occur with disease such as parvovirus, distemper, herpesvirus and pseudorabies.
I imagine your vet gave you Ruby an antibiotic, you should also feed a good quality canned food mixed with water to make a mush. Be sure to follow the antibiotic up with probiotics to replenish the good bacteria in the gut.
I hope rudy recovers quickly, she is adorable