When can an Anenome be added?

[Rikko]

If he's dead set on an anemone I'd try and steer him towards a bubble tip (Entacmaea quadricolor) as they're probably the hardiest I've encountered. They have very low light requirements compared to other anemones (at our other store we got a few from a customer because his kept splitting and growing out of control in his tank lit by NO fluorescents) - VHO or T5 will work fine for these guys. They also have less of a tendency to wander around and sting things and eat fish.
I don't necessarily agree with the "how old a tank needs to be" doctrine (I make it a point to argue any absolute number given in the hobby... How much water to change and how often is probably my biggest pet peeve), but if it's his first tank he should definitely hold off for the reasons BMW gave: he'll need to know exactly what he's doing. I've set up a couple of small displays and added an anemone only a few days later.. They always sulked for a day and then bounced back and settled in very well. I definitely wouldn't recommend that to a beginning reefer, but my point is simply that as long as you know what you're doing you can break a lot of "rules" that are in place to help the neophyte.