No. A jill (female ferret) will not die if it not bred from, but it will become very ill (and possibly die) if it is not brought out of season. A jill will stay in season for many months and this will lead to problems such as infection, which can be severe enough to cause death.
There are a number of ways to bring a jill out of season, the most effective of which is having her spayed. She could also be taken to a vasectomised hob so that he will mate her and (in theory) bring her out of season without her producing a litter. However, this method is not fool proof and can present different problems. Firstly, if the vasectomised hob is not proven he may not actually mate the jill. Secondly, if he has been used to mate other jills to bring them out of season there is the risk of cross-infection. Care must also be taken to ensure that the hob was vasectomised at least six months ago because they have the ability to store sperm for quite some time after the operation, which may result in an unwanted litter of kits.
The third option is the ‘jill jab’. This is a hormonal injection administered by the vet. However, there is always a chance that the jill will come back into season after being jabbed so by far the most effective and safe option is to have the animal spayed. This will eliminate the risk of pyometra infection of the womb.