While I don't agree with the idea of "losing friends", I do agree with the quote. I recently experienced a huge change of environment (different country, different people I'm spending time with) and I can clearly see the impact it had on me.

Interesting thing, the effect is somewhat "laggy". I spent few months with a very hardworking person but I didn't really start following in his footsteps until after 3 months. When I stopped interacting with him, I kept my new-found diligence for 6 months before starting taking things easier again (though I can say I do work harder now than a year ago).

During my first few months here I also started paying more attention to punctuality. But when I started spending more time with my constantly-late friend I lost it somewhere along the way.

So yeah, people around you do affect you in many little ways. It's possible you don't see it, because the change is usually small and happens over time. Would I select my friends basing on their success? Hell no.