Feliz Cumple'

Feliz Cumple'
 

I'm thinking of saying this to a close friend who is from Colombia, and just wonder how acceptable is its usage as an informal Happy Birthday greeting. The reason is that my friend doesn't like to be reminded of how many años she already has completed! Also, should it be written with or without the apostrophe at the end?

Without the apostrophe. It's a very common informal greeting. :)