Probably the best program might be Asiamiles. It's only because Qualiflyer will cease to exist by April - but their mileage redemptions are quite good.

Asiamiles bases awards on distance. So it might be better to take advantage of that and redeem a few awards inter europe.

But if your parents' are not mileage nuts like the rest of us, enrol them in a program that requires the least amount of explanation.