Based off this one picture, I'm not surprised she didn't test positive for tobiano. The white isn't crossing over the spine behind the saddle pad and there doesn't appear to be any white in the tail, either. If she has white crossing the spine where the saddle pad is, then I can definitely understand the confusion, but I can't tell from this picture
To me she looks frame (which you've already confirmed) and sabino.
Remember that the other key to preventing a lethal white foal is making sure the stallion is tested negative for it as well. If the stallion is of a breed that carries it, don't take the owner's word for it: demand test results. Friesians don't carry frame to my knowledge, so that would be a safe breeding.