That DPS didn't know anything about being a healer.

Healers in FFXIV are just dps with boring rotations that sometimes throw heals, in fact healer damage is so high that can deal more dps than tanks.

I have always kept my party members up at 90% health or higher
Side note: Don't be afraid to let your party members go lower than that, you'll rarely find high unavoidable damage in casual content and at 90% hp most of the heals are overheal.