Okay, before I pass out....
If your uncle feeds live and your snakes typically get f/ t you have a fantastic way right there to traumatize the snake. That much weight loss could be a variety of things. Because you gave your snake to someone with a bunch of snakesnthat don't get cared for in the ideal sense a likely cause is an infection, and tons of strains are possible candidates. Also I am going to go out on a limb and assume you don't track the humidity because you didn't comment on it. Just because a snake had a shed you may have perceived to have gone well, doesn't mean it isn't becoming dehydrated. Reptiles are generally better at retaining water than mammals because of beta kerratin in their skin (different than beta carotene). But this makes them susceptible to long term dehydration and kidney failure. They show the negative signs only slightly until it is too late. This makes me think you may have a dehydrated snake as well. Don't assume you can fix the whole situation by hydrating it because that is also likely a symptom. At this point figure out how much intervention you provide for the snake and do it ASAP. Waiting at this point doesn't serve any purpose.
I don't want to play captain hindsight or lecture you, but these arent play things. They aren't science projects. They especially aren't things to lend to just anyone. These are animals that depend on a constant assurance of maintenance and stability. Sometimes breeding projects are done without any concern for the animals doing the actual work. In fact, breeding is probably the single most dangerous event in the average captive ball's life. The amount of unreplenished calories in deficit is staggering. Even for wild snakes breeding poses countless risks. If you cant provide the minimum requirements for your animals what urged you to breed? I am not coming out against the practice of breeding but standards must be ensured.
Basically I don't know how much business you or your uncle have in breeding snakes. I am sorry for the rant but this was probably going to he said by someone else.