The fact that you already have a job should not deter you from taking interviews with other companies, especially the ones that come from a friend's recommendation.
The fact that your friend referred you to his employer without asking your permission first is something you should talk about with your friend. Most likely he misinterpreted something you said about your current job or wanted to help you in some way. Apparently he didn't bother asking first. Maybe you should make him aware of that.
Combining these two opinions, I would suggest you at least have a prolonged informal chat with your would-be employer. See what his offer is, check if it matches or exceeds your current position (only you could quantify the aspects that are important for this decision) and take a decision after that.
TL;DR: Talk to the guy that called you, don't make promises, don't sign papers, just listen to their offer and make a decision after. And tell your friend to ask permission before making a recommendation.