i echo the previous statements. if it’s not absolutely crucial for it to be a low D, i’d recommend an A, G or F, which ever you can get a hold of cheapest, though the F will be the biggest stretch.
even a few months on a mezzo whistle like those will be helpful for getting the feel of piper’s grip (which i use for A and below, and have small/medium hands [I’m 5’5"]) then try a low E or D
I once saw the Jubilee/Bingamon low D recommended here a while back… Haven’t tried it myself, but mean to one day if I ever decide to try a low D (not any time soon, heh. I still have trouble with my B-flat).
Thanks for all the suggestions. It might be best if I could persuade her to settle for playing a G or A or even a Bb but that might be hard given the budget and the fact she’s heard my Goldie Low D. The suggestion about Domnahl na Gruen is also very welcome. I will report back when any decision has been reached.
I would say don’t even tell her she has to settle for a B-flat or G. Just tell her it’s a gateway drug. 
Start her on one of those to get used to a wider grip, and once she gets the idea, you could always try the low D then. (at least, that would be my plan-- get a G and work on that for a while, and once I’m used to that, then go for the D.) Perhaps she’ll even change her mind after trying them-- I know my B-flat has me despairing of ever even getting to a G, let alone low D, and I’m rethinking my consideration of trying some day! (I also have small hands and I struggle with piper’s grip even on the B-flat; and I’m not a teenager but I look like one, lol.)