Yes, you don't need blacks like I said, or even premium units for that matter. You can do just fine with 0 rolls by just raising your core units properly (starting with Iris and Bashira helps a lot), then slowly add event, gold rush and trading post units to your team. But if you want an easier progress or just want to have a black, might as well reroll for one when you just started out.

- - - Updated - - -

Yes, you don't need blacks like I said, or even premium units for that matter. You can do just fine with 0 rolls by just raising your core units properly (starting with Iris and Bashira helps a lot), then slowly add event, gold rush and trading post units to your team. But if you want an easier progress or just want to have a black, might as well reroll for one when you just started out.

- - - Updated - - -

Like you said, it's his call to make. For me as an f2p, when I play a gacha game, at some point I'm starting to think "oh man, I sure wish I had a <insert highest rarity> unit". So might as well get one from the start.