Hi, welcome to GoY, most ferns can be divided, but the time to divide varies, we would nee to know which fern you have, to be able to tell you when to divide them, there are about 10 thousand species of fern, but the good news is, they can all be propagated from spores,
{if you have the patience, and a heated greenhouse}, Derek.

Hi Derek, Thanks for your reply. The fern belongs to a friend and I don't know which fern she has - I'll try and find out. Either way, I'll pass on your answer and she may give division a go anyway. If she knows which fern it is, I'll update the question. Thanks again, David