As shown by this replay;
Mega-Kangaskhan using Return with Helping Hand from Cresselia - 42.8% damage on first hit, 20.9% on second
Mega-Kangaskhan using Return without Helping Hand from Cresselia - 27.3 % damage on first hit, 14.4% on second
27.3 x 1.5 (helping hand boost) = 40.95%. However we didn't factor in Damage Variation, so it's roughly the same as 42.8 if we think about it.
14.4 x 1.5 = 21.6% - If we factor in Damage Variation its more or less the same as 20.9%
However as to why I said technically, the reason I think this result is true is because of the wording of the ability "Parental Bond". Exactly what I believe is this;
Helping Hand in fact only directly boosts the FIRST attack - however the abilty Parental Bond means the second damage will do half the damage of the First Attack - so if the first attack is 1.5x the amount, the second attack will also be 1.5x the amount of what it would have normally done due to Ratio Laws.
If you didn't get the middle bit, don't worry about it. The end result is still the same as the power is boosted, regardless of why.