I routinely spike my mate's food with really spicy hot sauce
Why are you doing that to your friends food?
I think it is a funny prank to pull off. It is harmless.
What you are doing is not ok. Your friend may not like hot sauce at all or could be harmed by how spicy it is.
He can handle it. He is not a wimp.
Whether or not he can handle it is besides the point. If he wanted hot sauce he would put it on his food himself. It is not your job to do that for him.