What type of fabric is safe for my ferret to eat ?

No, there isn't any type of fabric that is safe for your ferret to eat. A lot of ferret kits go through this - two of mine did. They do grow out of it though. For now, just stick with using hammocks and other hanging things that she won't eat. Like your little girl, mine only ate the loose blankets and towels that laid on the cage floor, but they didn't touch the hammocks and hanging cubes. After a couple weeks of no blankets, add them back to her cage and see if she starts eating them again - if she does, take them back out for awhile; if she doesn't, then she should be over the fabric eating phase (just keep your eye on the blankets in case she would happen to start up again). It took my one about two weeks to get over this phase, but the other one took about a month (she continues to have a thing for eating tags though, so I always have to make sure I cut those off before she gets to them!). You might want to give her some type of hairball gel/ferret lax or about 1tsp of 100% pure pumpkin (NOT pie filling - it should be pure pumpkin) after you've noticed her eating the fabric to make sure everything goes through and comes out the other end.