In nearly all cases an alternator comes with a pulley.
In this case it may have come with the wrong pulley and if true would make it the mechanic's problem. He should have known to check such a thing before installation as wrong pulley is a very common problem.
He should eat the cost of installing the correct pulley and the new belt.
Be alert. The world need more lerts...
The Ogre's Fiero Cave (It's also at the top of every forum page...)