A 777 is smoother-riding than a 747? I haven't noticed that, but even if true, I doubt that subtle differences would be relevant to someone who has trouble sleeping due to turbulence. I'd suggest they visit their physician and see what might be prescribed to knock them out for a bit.

As for turbulence being something you get used to, maybe for some, but not all. Some have never been bothered by it (a camp I'm in) while others will anxiously clutch the armwrest at the slightest turbulence even after years of flying. For me, I can't sleep on a plane unless there is a bit of turbulence. The steady unbroken droning causes me to be restless. Go figure.

Maybe the best bet, aside from a prescription, would be to not get anxious about not sleeping and make sure they've got something to pass the time with. Get them familiar with the movies that will be shown ahead of time and see if any are of interest to them. Chances are better than average that, if they get comfortable and distracted, they may get tired and just doze off.