Does your friend have special training with knives, or does he have any other reason for having a knife on him?

If not, you should get your friend a pepper spray (just make sure that whatever pepper spray you get him, it's legal in your state). But if he still insists on carrying a knife and brandishing it to warn off others (despite owning a pepper spray), I suspect that his stupidity is completely beyond repair.

I know many people that would feel perfectly justified in killing someone who brandished a knife on them (because to most people, a knife is a very direct threat to their lives or to the lives of their companions). And if he has no training with knives, that makes him even more of a tempting target to someone who is trained (or to someone who has a tire iron or a baseball bat in the trunk of his car), because only trained individuals know how to brandish knives correctly (and contrary to popular beliefs, what is shown in Hollywood action movies is not how you're supposed to fight with a knife either).