Can you reset an iphone 5s with the find my iphone app activate?

I found an iphone and it has a password put in and i'm trying to reset it but a friend told me that while the find my iphone is activate in the phone i found it would be impossible to reset it

You friend is correct. The best course of action:

Option 1: Turn the phone into the local police department.

Option 2: If the phone has a service provider's logo on it, then turn the phone in to that service provider. They can identify the owner through various identifiers inside the phone and return the phone to the owner.

Correct, its a security feature. It also notifies the owner where the phone is located when you have it on.

You can't reset the phone without the rightful owner's password. It's a security feature, designed to keep you from stealing the phone (as you've done) instead of turning it in to police. The phone is essentially a brick unless you have the Apple ID password required to reset it.