But use a non-detergent shampoo. Something very gentle that won't take out the oil. Last week it was fine, suddenly this week it is not using one back leg, nor eating much, huddled in corner and not voiding much. Vet suggested cutting off leg or putting it to sleep. You gotta imagine if this is a bony tumor of the femur.
Bony tumors are the bad boys of cancer.
It's painful, unrelenting, and will spread. And we're talking about a critter who has a limited lifespan. I would really evaluate the situation before I put a guinea pig through an amputation. I cannot even think of a time that I've heard of a guinea pig getting one. It's probably in a lot of pain and it may be time to be thinking about making that decision, to say that enough's enough.