A friend of mine has two JRs, one 2 years and the other 5 months.  Her OH took them out today and the little one decided to disappear down a couple of fox holes.  Fortunately, the puppy came back out after a short while, but my friend is very worried that this will happen again.

Obviously, its terrier instinct to do this, but is there anything she can do to deter it as she'd rather not just keep her lead up all the time.

Only one thing will stop them I'm afraid, training and more training, get them used to a clicker or whistle for recall, stop, stay, down, etc, buy your friend a book and a clicker/whistle and get her going with them, some breeds I feel co-operate much better by tools than our voices, a clicker or whistle can't be gotten around or ignored or not said in the correct strength of stern voice of 'not messing, I mean it!' A dog hears it and understands instruction immediately once taught.

Distraction can be used as well when your friend or her hubby see the dogs catching wind of something, sniffing the ground, looking as though they are after something interesting recall quickly and lead until you get passed. Keep the smaller on a long line until the recall is good.

Not only can the dogs be hurt by a bite from a fox/badger, but it is illegal for them to be hunting/killing/hurting them if they should happen to find one and if they go near when cubs are around a fox will defend just as any other animal will.

We have to keep our dogs under control as best we can, the dogs need training, otherwise they will obey their own instincts and you can't blame them for that.

Thanks for this Carrington.  You're right training, training, training is the answer.  They were talking about using one of those spray collars, albeit reluctantly, but I think the clicker/whistle is a much better idea.