It wouldn't be my first choice of places to go when looking to rehome a dog. Is there a reason why she isn't looking at a rescue organisation? Jack Russells aren't exactly rare in rescues.
I would honestly want the dog independently assessed by a dog professional - because that's what happens when a dog goes into a rescue like Dogs Trust. I'm talking about someone appropriately qualified - an APDT trainer would be a good place to start.
If she really does want to go ahead, I wouldn't accept anything less than a "warts and all" discussion about the dog. Make sure you get vet records. Make sure you take the dog out and about and see what it's like in different situations - around traffic, in the car, with strange dogs, walking on a lead, etc etc. There are many, many dogs that will look like perfect dogs in the house, but if they see a specific trigger, such as another dog (or strangers, or motorbikes, or men wearing hats, or one specific dog breed), you will rapidly discover that the dog has some serious underlying behaviour problems. I should know - I have a Jack Russell X who is perfect in the house and park, and a nightmare as soon as a motorbike comes into view (people have crossed the road to avoid us). The technical term for it is reactivity, and while I can cope with my dog's mild version, for some people and dogs it's overwhelming, and dangerous.
Get the owner to show you the commands the dog understands - for instance, there are several different words people use for down (and I once, memorably, met a dog owned by an elderly English lady who had exclusively trained him in Swahili!), some use a mixture of hand signals and words, some use exclusively hand signals, some use clickers or a specific marker word... You get the picture. It will also reveal if the dog is totally untrained.
I'm sure I'll think of some other things later on - but I would also seriously urge her to consider going via a rescue where the dog will have been independently assessed, and there's backup if you have problems.