How old is the little guy or girl, and what was the the activity before the limping? Limping can be brought on by many things just like with people, it depends on what they were doing prior. However, unlike with people pets can't tell you where it hurts. If you wish to give me more information, please feel free to do so. If not, here are some of the things it could be... simplest case, It could be a broken toe-nail. They are easy to spot, and if that is the case then take the little guy or girl to your vet and let them trim. Also, Schnauzers have hair that is thin, unruly and picks up objects like a magnet. If they are not kept well groomed, especially the pads of their feet, their hair becomes knotted and can entwine any object... stick, stone, etc. If this is the case it could be a burr lodged within the pad.
If none of the above applies, and it's a puppy, most likely it's just a sprain. Like children, they are made of rubber and tend to recover quickly. Just keep him/her quiet as possible. As a general rule Schnauzers are not prone to hip problems, so if the pet is adult or a bit older, your friend is most likely looking at a cruciate injury. That is a ligament in the knee. In the human field it is one of the most common injuries among athletes. In the medical field (human or otherwise) it is commonly known as an ACL injury. Depending on its severity, surgery may be warranted. If that is not in one's budget... the body will heal itself, and though the little guy or girl will have arthritic issues, there are supplements that can be given to help.
If a day or two of rest doesn't help the little guy or girl, please take him/her to your vet. But I hope your friend takes this going in more informed. Any other questions or concerns, feel free to write... I am more than happy to help.