I'm sorry that you got robbed. Oh well, the past is there to teach us a lesson or two in life. Thank God I've never been robbed by a friend (though I did get one of my DBZ action figures stolen back when I was 10), but yeah.
My advices are:
1. Don't trust anyone you just met, seriously man. Hang out with them in much safer, public areas, but keep your guard up at all costs. Observe their actions and words to see if they really are being true to themselves. If they seem harmless and lower their guards down, then you may do the same. It's all a matter of trusting the right person that you'd be most confident with.
2. Communicate with their folks and/or any other members of their family (good milk comes from a good cow) so you can know more about the guy's lifestyle and you can ultimately make up your mind to remain friends. I would never want to be friends with anyone who does drugs, is an alcoholic, and/or is a criminal, (ie: thief, gang member, hitman).