What are some good exercises that someone can do at home for a torn acl?

Knee motion & quads. Most ACL tears may require reconstruction. Early home therapy is routinely geared toward preparing your lower extremity for surgery. Quadriceps exercises are excellent. Simple repeated bending and straightening your knee once tolerable also helps decrease swelling, keeps the joint from getting stiff, and keeps the cartilage or joint lining healthy. The default position at rest is full extension.
Quad strengthening. I assume you mean a torn acl that has not been repaired. The best focus is building up your quadraceps. This can include wall sits, straight leg raises, biking, and eliptical. Avoid lunges and knee extensions. Any instability issues should be addressed with an orthopaedic surgeon to determine if surgical stabilization is required. Good luck!