No statute on point that I'm aware of. You will be caused to sue on your contract and assert there was an agreement regarding the seller fees.
NOTE: The use of the Internet for communications with the firm or this attorney will not establish an attorney-client relationship and messages containing confidential or time-sensitive information should not be sent.
There is no law that requires your friend to pay you 1/2 the value of a real estate commission when no realtor is involved.
Some have used that tactic as a negotiating point. However, the other side could call your bluff and say go ahead and list the property for sale and pay the real estate commission.
If this answer was helpful, please mark it as helpful or as a best answer. This answer is for general education purposes only. It neither creates an attorney-client relationship nor provides legal guidance or advice. The answer is based on the limited information provided and the answer might be different had additional information been provided. You should consult an attorney.