Adding water after fermentation starts

Hi all,
I have a friend that brewed his first batch last night. He forgot to top off the carboy before pitching the yeast. he was short about a gallon of water. would it be safe to top off if the yeast is already active? should he add boiled water? Tap water? does it really matter?

not sure what to tell him, never had that issue.

thanks for the help

I wouldn't personally. Did YOU take a hydrometer reading before YOU pitched YOUR yeast? (friend, yeah right)

If you hit your numbers then I would rather have 4 gallons of good beer than 5 of watery....or if you didn't take a hydro reading, then you will simply have 4 gallons of stronger beer.

If you add water at this stage, you are opening yourself to several possible problems, such as contamination, and/or oxygenation.

I dont think HE took a hydrometer reading. dont think he has one. Thanks for the post. after all stronger is better.

thanks