No you will loose fluid but air wont get trapped in the system if you do it this way. Have him pump the brakes, then hold it down. Once you open the bleeder it will shoot fluid out, but it will stop. Then just change out the bleeder and you wont have to bleed the brakes afterwards.

Fill the reservoir first, and tell him to not use much pressure on the pedal at all, or he will end up hitting the stop on the pedal before you have the new valve in. That would mean a bleeding job would be in your future.
You can get the valve at Autozone, I'm told. If not, try the other similar parts stores, and the junkyard last.
This should be a pretty easy job, I doubt you will need to bleed them after doing it. I think the bleed valve is metric but I'm not certain.