Can you post a photo of the horse taken from the front?
How serious being toed in depends on if the knees are 'normal' or not.
Many toed in horses also have 'bench knees' or 'off-set cannons'. When viewed from the front, the forearm should attach into the center of the knee. Then, the knee should be straight (not tilted toward the inside) and the cannon bone should be absolutely straight underneath the center of the knee. If you drop a plum-bob from the center of the forearm, it should go directly through the center of the knee and the cannon bone, the ankle and, ideally, the hoof.
If only the hoof is toed in and the knees and cannon bone are where they should be (not off-set at all), the slight toe-in will not be a problem.
If the knees are not centered and the cannon bone comes out from the lateral aspect (outside) of the knee, the horse will not last very long without popping medial (inside) splints and arthritic knees or knee chips will soon follow. The long-term soundness of horses with off-set cannons and pigeon toes is not very great. They last a lot longer for light pleasure and trail riding than they do for a hard occupation like jumping or barrel racing or ????
The reason horses with off-set knees also have pigeon toes is that mother nature attempts to put the hoof under the knee to carry the horse. That puts the full weight of the horse on the inside part of the cannon bone and explains why they get medial splints.
Hope this helps.