He will need a plumber, preferably one that specializes in drains, so that the drain system can be checked. Maybe get referrals from Angie's List or the Better Business Bureau.
Sounds like it could be a clog in the main drainage system, which could be caused by tree roots or old pipes.
And be sure to get a couple of quotes, especially if the quote is high. Our neighbor was given quotes ranging from $250 to $1,500 for a similar problem. (It was tree roots.)