Is it true that if one has a crush on one of their 老友记 that one should not tell them? I've heard that it could ruin the friendship.

It's not a "fact". A lot of people are 老友记 with someone and then later end up marrying them. It's just if anything happens and it's a messy breakup it can ruin the friendship, but even if 你 do break up 你 can still continue being 老友记 if the two people don't act childish and bitter.