hi darren, about 2 years ago i found a little jill ferret wandering in my back garden in exactly the same physical condition that you describe. i didn't expect it to last the night and was all for ending it's misery there and then but my wife talked me out of it.she heaped loads of tlc on that little ferret and now it is fully recovered and happy living with 2 other jills.so yes yours should get over it's bad start and fair play to you for taking it in.my wife fed ours little and often,about 4 times a day.
regards graham