I would not suggest investing to Bitcoin using their save money, it is very risky. There is a golden rule that " invest what you afford to lose " it looks like your friends is going to lose money because of the unstable price of bitcoin. Or they will gain, its the investor risk to take. But if you are so eager to teach them just to give them knowledge I think its ok, like common knowledge, how does bitcoin work and some technical of bitcoin. Let them know that Bitcoin isn't about just gaining money.