I was going to say, if he's used to being one of two he may find it very hard to settle as an only dog. Often two are
less work than one as they keep each other company, play with each other on walks, etc...

I'd ask to have her 'walk you through' the dogs' days and nights - show you where they sleep, what they do between walks and feeds, etc, so you can gauge whether what this dog is used to tallies with how you envisage life with (a) dog(s).
For instance, if they sleep on her bed but you want a dog to sleep in the kitchen, with an adult dog already stressed by moving house, owner and companionship, this could be a hard one to manage.
Look for scratched doors, evidence of garden escapes, etc, and get her to talk about any 'management issues' as well as the dogs' good points!
And take it/them for a walk - some dogs pull constantly on a lead, some won't come when called, etc.
But broadly,
If her house & car aren't chewed to pieces & you like him & he likes you all will be fine! Why not take both?
this!
