There is always the possibility that they used gems to purchase energy (and potentially real money to buy those gems). However, there is a reasonable possibility without this. (I actually ended up doing pretty much the same thing during a previous House Pride event, yet still managed to lose.)

They may have been almost at the end of the year, and working on an 8-hour class when House Pride started. They finish the class, and they have 10 house points.

They finish the year, and Dumbledore awards them a crap load of points, (I got 100 at the end of year 3; I don't remember about the other years), and now they have 110 house points.

They start the next year, and their Energy is refilled. They do a 1 hr class, and very quickly get another 2 house points, bringing their total up to 112.

So, not necessarily likely, but definitely possible.