Some other account got added as my sub domain

Some other account got added as my sub domain

I was helping my friend with her domain mail setting. I had also sent a reference to my friend for bonus mails. By mistake I had verified my friend's mail which has come to my domain thinking its for acknowledging for bonus mails. Now I have my friend's domain added as a sub domain and all my friend's mail is getting delivered to my id. we want it as two separate domains and two different accounts. How do i do it.

Regards
Rema