As between A and your father the matter is ended and there is nothing more to be said about that.
As between B and your father if you read the agreement with B very carefully you will find a clause or sentence or reference which says in essence that if another agent introduces a purchaser during the period of the agreement then the client (your father) will have to pay B the fees and expences stated in the agreement. All agents agreements say this in some way. Your father should therefore pay up. A word of warning - if it goes to court although the amount may be within the Small Claims limit the agent will instruct solicitors and a barrister. Where a matter should not have proceeded to court, such as this, the small claims court can (and must) award a business its costs (irrespective of the "no costs" rule of small claims) so your father will be down a further �2000 at the very least. Even if B acknowledged existence of the letter to which you refer this would still be the situation, it would require a formal variation of the original agreement signed by both parties to make any change,