I'm sure there are guys here with more knowledge on this one, but I can say a couple of things:

From my own experience, reflexing the ailerons a tad can sometimes help delay a tip stall, so it's worth trying.

However, I personally think the reason "why" given to you is wrong. I think the help is because the raised ailerons reduce the AOA of the wing slightly, and the effect is more pronounced at the tip, since the ailerons are usually a greater % of wing chord at the tip than the root. So you basically are adding washout when you raise the ailerons.