I think technically it's illegal, but I don't see anything unethical about it. If this person is also discussing the hand with other people in the hand, THEN there's a problem. But as long as they only have the information you have, hell I don't see why a friend can't dictate what you do, it'd be the same as you sending him the money and letting him play. Unless the friend is also in the hand or has multiple "friends" playing at the table there's a problem. Note that this however is a cash game.
For tournaments, it's definitely unethical to have a person who's in the tournament (even if they busted out already) give advice on a hand. In fact, it was more than mere advice, but this got people to lose a lot of money they won, there's another thread around here somewhere on that (I think the WSOOP but I'm not positive).