If I found out my friend is a vampire, it would be the beginning of a series of well calculated steps.
Of course, I would be shocked at first. Not just shocked, I would be worried. I would wonder how many times my friend has seen me as potential meal. Vampires feed on blood and that would be concern to me.
I would wonder what secret plans my friend may have for me. I would question if the friendship is actually a genuine one.
Another concern of mine would be how many people this friend has fed on. I've read it's difficult sometimes for vampires to control their hunger for blood. I would want the safety of my family as soon as possible. I would stop inviting my friend to my house because it could be a risk.
Now, regarding what I would do after finding out if my friend is a vampire. First I would try to get evidence. This is so that whomever I tell about this believes me. I know it may be hard and risky, but I would be very careful and discreet while trying to gather evidence.
After that, I would report him to people and the authorities. This is for the safety of the public. They should be aware of such a thing so they don't fall a victim.
The authorities should be able to handle it. I know it would be heart breaking reporting my friend but the greater good is important. No one knows how much danger people would be exposed to. I don't want to be in a situation where I had the chance to warn people about the existence of a vampire in the vicinity and I didn't. Humans and vampires shouldn't stay together. It's like leaving yam and goat together.
The good thing about reporting to the authorities is that my friend would be better handled by the authorities than facing a mob action. If the people finds out that he's a vampire, he may face a mob action.
The next goal would be to fish out other vampires in hiding.