My friend and I are hanging out tonight and I want them to have a good time, so I'm going to put more alcohol than usual in their drink without telling them.
That does not sound like a good idea. Why would you want them to be drunk?
If a few drinks could turn them around, it would be a good way to make my friend feel better.
Sneakily adding alcohol onto your friends drink is not only dangerous but can be very unethical. What if someone would take advantage of their drunkness and sexually abuse them? Why not just enjoy your time with your friends, and not worry about their alcohol consumption and focus on having fun instead?
Sometimes I plan my out of control behavior, but for friends it's always for their own benefits.
Your friend probably would drink that much if they wanted to, but they clearly didn't and so you shouldn't try to trick them into it. You may think it is for their own benefit, but it could end up backfiring and hurting them greatly. Speak with them first before you do anything.