I have seen charging instructions that say to fully charge the battery
before first use.

A friend of mine just bought a cell phone. He took it home and
plugged into the charger and went online to activate it. He never
once looked at the manual.

I tried to tell him he should charge the phone fully first. He said
that only applies to devices that only work on battery only. Because
he had the phone plugged into the wall that it was ok to use right
away.

Who is right?