Hello, something has been bothering a friend of mine lately. It's a character at the end of ACoS, who helped Rand kill Sammael in Shadar Logoth. I speculated that this might be Moridin, since he seems to want Rand to survive until Tarmon Gaidon, but I'm not certain. Do you have any idea who it might be?

It is Moridin, confirmed several times in the later books.

A number of threads discussed this.

 

The mindtrap chapter seems to also confirm.