I was told by the owner that she had NOT been tested because her sire was not a carrier and that if the sire isn't, the foal shouldn't be either, regardless of the dam.
Warning...HYPP discussions tend to get very emotional.
In HYPP info, you will see H/H, N/N, and N/H.
Regular genetic rules apply and it is true the foals cannot be H/H, but they could inherit one HYPP gene from the dam.
H/H means the horse has inherited the HYPP gene from both parents. You definately want to stay away from this, but if the sire is not a carrier, the foals cannot by H/H.
The more emotional discussion revolves around N/H horses, where they have an HYPP gene from one parent. I suggest you research unbiased literature (e.g. from vet schools) about this for your decision. From what I've read, many N/H horses never get symptoms at all, to some with mild/rare symptoms, to some with more, and the key is managing their diet. Part of the problem is that there are no real statistics about how many horses are affected and how.
Finally, of course the N/N horse that does not have any HYPP genes.
There is an HYPP overview on the AQHA website that includes research references at AQHA - HYPP INFORMATION
If you are concerned in the least, I would pay the $40-$50 to have the mare tested, so you won't be wondering about it. I certainly wouldn't walk away from a good horse just because it was Impressive bred.
BTW, our Paint mare, Lady, is Impressive bred, and is N/N.