Chances are the bunny had a lethal amount of Vit C in his system.
Rabbits and Guinia Pigs should never be living together.
Both have Specific feed requirements the guina pig having a diet of High Vit C.
Guinia pigs can not produce Vit C naturally, and have to be supplimented through feed or/and fruits and veggies.
If the feed was a Guinia Pig blend then chances are there was too much Vitimin C in the rabbits diet resulting in a Toxic amount.
""Extreme excesses of Vitamin C can cause kidney damage.
Excess vitamin D can cause calcium deposits in tissues (joint, kidneys, etc.).
Excess vitamin A causes neurological and skin damage.""
I am sorry the bunny passed away .
I am more than aggravated with the vet Services in your area.
I would be on hte phone and throwing a level 10 about the lack of concern these vets showed.
there is no excuse for their behavior.