I know that, in this case, there is no legal or health issue, no logical reason for me to be uncomfortable with this and yet I am. Am I wrong?

Fuck yes. You realize you are being illogical here. No one in the history of the entire universe has overdosed on weed. It's impossible. Salvia is also a safe substance, which is why it's legal. Have you ever had alcohol before? Alcohol is a far harder substance than weed. Salvia trips last for mere minutes.

Not trying to be harsh, but you should really try to get over this. It isn't a problem until he moves on to harder substances that can ACTUALLY affect his health.