My friend has a JRT whom she rescued from Gypsy's, she is a lovely little dog, affectionate and good in the house but she can't let her off the lead as she just runs. Recently she has bringing her to our paddocks where we keep our horses and has been let off as it it is fairly enclosed and she has been running around and coming back which was great BUT twice she has got herself stuck down rabbit warrens and both times we have dug her out. Now my friend is back to square one as she now daredn't let her off . Any ideas would be great, she isn't food orientated at all so treats don't work. She has obviously been trained to this but only being about 12 months old can she be retrained and how??

I have a JRT now and have had in the past.   I would stop letting off and have on a long line and restart basic recall training.  Join a dog club etc so dog can learn to recall even in exciting situations.
The more the dog is let off until you are certain it will come back is the more it is likey to get ingrained in its mind it can run off and ignore if it wishes.
Quite a few get lost forever going down rabbit holes as the underground track can go for miles and they could end up some distance away before getting stuck and unable to get out so you wouldn't find thier location to dig them out.  The best scenario then is that they starve and dehydrate losing so much weight they can find an exit out then.
JRT are very strong willed dogs and some are easy off the lead and very obedient with training and some just never really respond well enough to be trusted off the lead.
I exercise my pup on an extending lead (the extra long one) and he is showing all the signs that he may be one of those that always has to take his exercise on that lead.  But its early days yet but I would rather be safe than sorry.

Bev