I am pet sitting a ferret.. how do I deal with the smell?

Unfortunately, since she's not yours, there isn't much you can do.

Normally, a ferret fed a high quality food with a clean cage and litter box will NEVER EVER smell, so she must be on some nasty food to smell that way. You can't really change her food since she isn't your ferret. All I can suggest is use a high quality litter like Carefresh, Yesterday's News, or Feline Pine. Clean the litter box every day, and after a week wash her blankets and towels in the washing machine. If she still smells, then it's the food causing her to be like that, and talk to her owner after about changing her food to a better quality one.

Clean the litter box daily, wash all bedding, and do NOT give her a bath.

Bathing a ferret washes all the oils off it's body. It will then go into overdrive to replace these oils and smell worse than before you gave the bath.

There is not a single animal that smells good. Most of them are smelly, my guinea pigs smells too but we bathe them occasionally,so,try and ask the owner of this ferret if you could bathe him/her. The smell might come from the mixture of poop and pee.Do you clean it's cage??

its a skunk

and stop sitting on it