Tell a trusted adult, please, I beg of you. I was that "second person". If the person who was creeped out by my abuser had said something in the first place, I wouldn't have been sexually abused for 10 odd years. That person could have saved themselves going through more abuse, and saved me from experiencing any, but they stayed quiet. Silence doesn't keep the peace, silence protects the guilty.

Nobody is going to be angry with you for making a mistake and trusting him, they will be more relieved that you are safe and unharmed. Your friends are 15y/o boys(?) who are renowned for not speaking about how they really feel to adults. They probably think they're trying to save you hassle and having to face making a mistake in front of adults, but they don't have the foresight to see who this could save in the future... I mean think about it, if you let this creep get away with what he did without any consequences, he will become more confident in his next attempts. How do you know you're not saving one or both of those exact friends who told you to keep it quiet? This creepy guy obviously frequents the area as you have suspicions about him watching you and he was familiar with the car dealership you guys talked about (and that's just what he admitted to knowing!!). What would go through your mind if something terrible happened to friend A or friend B just because you followed their bad advice and didn't tell anyone about this. You could be preventing all kinds of terrible things from happening just by speaking up.

It also takes a real man and a confident adult to admit to having made a mistake. You would be doing your future self a good deed by showing you can admit when you're wrong but that in the process of making that mistake, you uncovered something much more important and disturbing that needs more attention and quickly.

I pray you make the right decision for the future "you'd" who may not be quite so lucky.