If I'm dealing drugs and an informent comes into my house to buy some from , are they legally allowed to smoke drugs with me at the scene?

From what you have written, if you are under suspicion by law enforcement for the distribution of controlled substances an informant within the course and scope of the police investigaiton is allowed per the terms of the agreement he or she may have with law enforcement to purchase the illegal substance from you as well as ingest it in your presence where the transaction occurs. Such would be legal under the laws of all states in this country.