They will never approach a visitor just for being friendly. So get real.
Utter and complete nonsense. While the advice to be wary of "over-friendly" strangers is valid ("HEY BUDDY! You American right! You want girls! Let's go my house!"), you are depriving yourself of serious traveling joy with this attitude. I've been approached by countless people in this way... maybe everyone has their "motive" – i.e. getting to know you – but this has resulted in invitations to small town fiestas, late-night concerts, family dinners, and more.
In this case, the "extroverted" Canadian was up for adventure, and they ended up at the San Marcos Feria, which is really a pretty fantastic event that many foreigners would be thrilled to end up at. A phone was pickpocketed at the country's largest festival – yes, this is shocking, but turning it into advice to never chat with new people really isn't the lesson here.