How do you get Golbat's Friendship level up in HeartGold and SoulSilver?

Answer 1

Let it hold a Soothe Bell and battle or do other things with it. There are other ways like keeping it in party, but you know you don't need the Soothe Bell. The Soothe Bell only do is increase the happiness faster. You can give it some berries that contains the heighten ability of friendship to it, but the bad part about it is that one of the Pokemon's stats can be lowered.

Hope this helped!