My answer is based on whether or not the 500 is not changed much from the European model that has sold for quite some time in Europe.
If it is the same model, then it is one of the most popular small sized selling vehicles in Europe.
I'm curious how your friend would know it is prone to mechanical problems when it hasn't been on the US market long enough to make that determination.
Before you start comparing the smaller vehicles like this, understand that for each one, you will find hoards of complaints on all of them.
So the best advice I can give you is to take all of the information you get from articles, friends "that know" and all of the other garbage out there, and throw most of it away and make your decision based on what you have found out for fact.
Test drive one. Put it through it's paces. I know someone who bought one and hasn't had any problems with it. Is that the norm? I don't know. As I said, it's life span here isn't enough to make that judgement. I do know that the European mechanics I deal with don't have anything to say about a lot of problems with the ones they deal with, other than they are small.