Do you need to strip it completely? What about protecting threads? Do you need to take off headsets, bottom brackets etc? Will they strip off the original paint for you?

A friend wants to do it for his wife's bike, so I need some advice please.

depends what you ask for, blast and paint or just paint.

I'd protect the bb threads with a set of old bb cups.

you'll need to remove EVERYTHING

In that case, how hard is it to get the headset out?

Also what if I don't have some old BB cups?

If you've not taken a headset out before then maybe best to get hold of some one who has to do it for you.

your LBS may well have some cups, pop down and tell them why you need them.