Mine was a bit tongue in cheek... The way you're "supposed" to do this investing thing is first, decide on what your asset allocation is, then keep it balanced to match the allocation.
If his target for the asset class is 3%, and now it's at 1.5%, my recommendation would be to buy another 1.5% from the asset classes that have "won".
If his target for the asset class is 3% and, looking for a big win, he instead had it at 20% and it went down to 10%, I'd say sell 7%. I think most people who recommend selling assume that this is the scenario, but I saw nothing that indicated for certain that this was closer to reality than the 3% down to 1.5% scenario.
I stick by the idea that the OP's asset allocation target is crucial to answering the question of whether to sell, buy more, or stand pat.