Is walking good for someone with a torn tendon in knee?

Yes, but... In general, a low impact activity that does not aggravate your symptoms is good for you. This does depend on which tendon is torn and if you have an unstable knee. If there is any instability, a brace should help you to walk comfortably until you are strong enough to go without it. Physical therapy can help maximize strength of surrounding muscles to compensate for the injured one.
May or may not. You are probably talking about either a torn meniscus or a torn ligament. Walking will most likely be uncomfortable and may sometimes cause more damage. Inactivity, however, will not fix the problem either. I would recommend less weight bearing and some exercises that are recommended for torn menisci and ligaments that you can find in abundance on line or through your primary care provider.

Related Questions

Fell of my bike and injured knee a week ago. It hurts to bend knee and lift leg when walking. Torn tendon? Deep muscle bruising? See Dr or wait?

Go see MD. After a week most minor injuries to the knee should be getting better. The possible injuries are extensive and need to be evaluated by an orthopedist, sports med doc or joint specialst (rheumatologist), depending on who you have access to. Stay off it a much as possible until it is checked out.