Hello, I'm sorry to hear about your friend's bunny and I'd like to help.
There are a couple reasons to have multiple under the skin nodules that are uncomfortable to the touch. The good news it that with multiple lumps cancer or a tumorous cause is unlikely.
The two most common causes are abscesses under the skin, usually due to a pasteurella infection or an infestation with cutrebra or bot fly larvae.
Pasteurella is a bacterial infection that is endemic (meaning it is always found even if not all bunnies appear sick) in some rabbitries. It can cause upper respiratory infection symptoms, pneumonia, eye infections, oral infections and abscesses, lymph node infections and abscesses under the skin. Once a bunny has the bacteria it is virtually impossible to get rid of it completely. We treat flareups appropriately depending upon the symptoms. That may include eye drops, oral or injectable antibiotics, and surgical removal of abscesses. Rabbits have inspissated or dry cheesy, purulent debris, so abscesses cannot be simply lanced and drained they must be curetted out or better yet removed entirely.
Bot flies or cuterebra are flies that lay their eggs near rabbit dens or cages. The larvae emerge, get into the bunny through the oral cavity or nasal passages and travel to various spots in the body, normally settling just under the skin so the larvae can emerge when mature through a breath hole. In some cases the larvae can travel to other spots such as the brain or spinal cord or into the abdomen or lungs and cause serious problems. The larvae must be removed via a quick surgery as if we attempt to remove them via their breath holes and the larvae ruptures that can lead to a horrible anaphylactic type reaction.
What sort of lab testing was done?
Was it an aspirate or biopsy of one of the lumps? If so that should give a diagnosis and direction on therapy.
In the meantime I would look for breath holes associated with the lumps. Here are some pictures:
Make sure the bunny continues to eat well as either one of these disease processes burn lots of calories.
Please let me know if you have any further questions.