I think it depends on who, how, and in what context.
One of those minute but highly complicated judgements we make everyday.
If your good friend happens to be in a solid relationship, and you also trust them, it can be a compliment - ettiquite then demands a similar response from you!
If you good friend happens to be your partner's best friend (apart from you
) There's probably little cause for concern, it's a friend thing.
If your good friend happens to be on top of your partner while saying this...there may a problem...
If it's just a 'mate' who's said this out of the blue...I'd probably be flattered and wary at the same time.