My brother/roomate broke the plastic knob off the shower handle and proceeded to use plyers to turn the shower off and on instead of telling me the knob broke. Now the stem has been pinched and the threads stripped. Any suggestions on an inexpensive way to repair or replace?

By "threads" do you mean the internal threads or the external grooves that keep the handle from turning? The simplest thing is to get a new faucet stem. However, drilling and tapping an internal thread is fairly easy. If the outside is stripped, I'd get a new stem. You can try and epoxy the old handle on but that's not a very good fix.

They aren't "threads". They are called "splines" and if all he did was to squeeze them together all you need to do is spread them apart and purchase a new handle. They also sell replacement handles and knobs with set screws if the handle slips on the stem. Good luck, Tom