Tipping a friend who is a bartender

I have a friend who is a bartender at a very nice restaurant. The wife and I went to dinner and ate at the bar, as it is more low key there and we can visit with our friend and people watch. When the bill came, my friend basically comped us all our drinks and only charged us for food. All the drinks he brought us were items he thought would pair well with our food. In other words, he didn't ask us what we wanted, but knew what would be good. No telling how much the bill would have been had he charged us, but I left him a 40% tip on what he did charge us. Wondering if I should have left him a bigger tip, and what the protocol is in situations like this. I didn't want to leave a 100% tip because I thought that would look suspicious and my friend wouldn't expect something like that.

This is our first time to ever take advantage of this situation, and we will not be doing it regularly. I am interested to see the different responses to this.