A good OC works fine. Folks will come along and say it dosen't work all the time etc. and so forth. We should remember that guns don't stop the threat all the time either. You friend should gain some understanding of dog behavior, and take along a stick and OC. I would prefer a good sized can that gives a longer range stream.
"A Liberal is someone who doesn't care what you do, as long as it's mandatory". - Charles Krauthammer