This could be a good opportunity for him to step up and get a nice center
Not for a $100, unless he finds a good used one for that price.
The only other choice would be Harman Audio's ebay store, where he will find auctions for Infinity and JBL centers. He might get lucky and get an auction for around a $100.
But even then, might only get a cheaper 2-way center, instead of a higher priced, and much better 3-way center.