I don't know where you could try things on in person, though the hospy physios I saw during my first pg sold belts, as well at tubigrip (fantastic that! just make sure you hem it with an overlocker or it will fray).
I'd urge you also to have a look at the Pelvic Instability Association (Australian) website that you can google to see if she has any of these symptoms (in which case it's not about the size of the belly at all, because mine is a poor excuse for 34 weeks and I've had SPD for 3.5 years since my first pg and it's back with a vengeance ).
For the pain, I would highly recommend (as well as wearing a brace/band) acupuncture to complement the osteo.
I would also recommend the purchase of one or two Belly Bras (made by FertileMind) - available on Mothers Direct if MelanieK (google these, too), a shop in Melbourne with an online store as well, doesn't have any in stock (Mothers Direct has them for $69, or less if you are a subscriber to ABA, and MelanieK has them for $20 for a limited time and I'm not sure when this price offer will end). Wearing it gives you a bit of a muffin top (around the sides only, it goes all the way down the belly to lift it off the pelvis), but hell, it's pregnancy and the aesthetic-police can bite me! It means I don't have to wear the tubigrip to get relief and the sacro-iliac belt I've borrowed only has to be used occasionally (mainly to take DS to Kindergym!) with it. Can't recommend this simple product enough - I'd have paid the $69 for it happily! I was even considering buying another belly belt thingy for about $90 before I received my BellyBra because of the hope for relief, just didn't need to in the end.
Anyhow, hopefully it's not SPD because that would be awful. Definitely the BellyBra, and the Mothers Direct retail outlet is in Malvern East if she needs to try one on before buying.
And don't forget acupuncture