The only thing a jack needs under the kennel club to be tested for is juvenile cataracts, some breeders also like to do hip tests while others don't as the condition is quite rare in the breed. Most breeders here will breed form the third heat onwards, small dogs like JRT are fully grown by the age of 1 year. Your friend needs to do a lot of research on breeding first, as they shouldn't even consider breeding if they don't know about heat cycles, then there are lots of other things to think about first, such as the cost and time involved and also the fact that you could loose the dog. So the first thing your friend needs to think about is, is her dog the standered has the dog proved that she is the standered by winning shows etc. Now there are also people who breeds JRT's as working dogs, In my opinion these dogs should still be tested for health and temperment, but dont need to be the perfect standered.
Also what type of JRT has she got, because things will be different with each type, also where does the person live as things are different here in the UK than in the states