As long as he pays the Realtor he signed the rep agreement their agreed upon commission it is fine. Hopefully the agent dotted all of their I's and crossed all of their T's in order for the document to be held up in court should it come to that point. I would first get with my broker to make sure he/she will back you up on this. Because if they don't, it may be a difficult fight to go alone. Not impossible, but challenging.
The Seller can sell his house to whom ever he wants, it's his house, but if he had entered into an Exclusive Listing Agreement with a Realtor, the Realtor has a right to demand his commission.
If a Seller signed an exclusive right to list the property with a broker/agent, then they are not supposed to. That would breach the contract. The Seller would still be liable to pay the broker/agent any compensation outlined in the contract at the sell of the property.