Ferrets can eat both ferret food and high quality cat foods. However, there's only specific brands of catfood that are just right for ferrets. I feed my ferrets a blend of ZuPreem Ferret Food as well as Eukanuba Chicken & Rice Adult Cat Food. I also have a little bit of Innova Evo Feline(they have canine and ferret versions too, which are just fine), which I sprinkle on top.
To be honest, ferrets don't generate an odor that fills a room. That's a misconception. They have a slight, musky odor that eventually becomes pretty pleasant to smell once you get used to it(almost all ferret owners will say this). You can only smell it two feet away at most. Anyways, the real deal is that ferrets are poop machines. They poop a lot, and based on their diet -- their poop can smell. I don't really notice it much myself, because I am very diligent about cleaning their living space.
There are some good foods that can cause really smelly poop, but the majority of bad foods cause it too. I usually settle for the good quality foods over the smell because I clean often.
Gosh, that's horrible about the baths. I've ever given my ferrets a real bath. There's no point to unless they get into something really icky. It's bad to bathe them that often because 1) it can dry out their skin and 2) ferrets will produce even more musk oils to compensate for the ones lost during the bath. This can cause their natural smell to become much stronger.
Btw, I don't mean to be a downer, but ferrets in general are really difficult animals to adjust to for most people. You've got to ferret proof the entire area because they'll get into EVERYTHING. That, on top of the frequent pooping, high incidence of diseases later on life, and difficulty with litter training in a new place(if ferrets have a huge area to explore, you'll have to litter train them in all those new areas too because they don't want to go far to find a box) makes ferrets rather troublesome at first...so most people end up giving them away. It would suck if your sister had to give them away as well since she has an infant to take care of. Does her boyfriend work a decent job to pay for vet bills? Ferrets can be expensive since they're "exotics."
Anyways, good luck to her. If you have any specific questions about food or treats or bedding or health or anything else let me know...and I'm sure some others here can help you as well.