Last week, we ferret sat my daughter ferret Ruby.  Ruby and our ferret Foofer got along great until Foofer got sick along with our other ferret Tinky.  Tinky didn't like Ruby and would biter her, so we kept them separated.   So for 4 days, Ruby had no one to play with but us.   Then when Foofer started feeling better, we let her play with Ruby, but Ruby was scared at first because Tinky was so mean to her.  After a while, they remembered each other and played.  Tinky had to remain caged.

So Ruby left last Saturday.  My girls were back to normal until yesterday.  Tinky started treating Foofy like Ruby.  Foofer loves everyone so it really ticks me off that Tinky is being such a royal diva.  So why all of a sudden is Tinky being such a meanie? 

(On a side note, after giving Tinky that Panacur her fur is so much softer.)

tinky could just be a little miffed that she was cagedwhile everyone else was out.
make sure you firmly say "no! be nice!" when she's being rough, and she should figure it out.
give it a few daysto see if her attitude changes. and keep disciplining bad behavior. consistency always gets me results, i hope this helps! =]

on a side not, now that i re-read what you wrote, you were saying foofer and tinky were sick?
ferrets can sometimes act aggressive if they are in pain, i would suggest carefully watching her, checking her poops, and feeling her stomache for bumbs n such. is she sweet to everyone else?

Tinky and Foofer played for 3 days after Ruby left.  I think she may be having a relapse.  She is not her normal self again.  Foofer is just fine, probably because she is younger than Tinky.  We are putting Tink back on her medicine.  If that doesn't help, we are going to the vet.  Poor little thing.