This is not a duplicate of Can you share digital games on Nintendo Switch like how it is for PS4 and Xbox One?, as that question was asked before much information was available about Nintendo Switch Online. This question is specifically about that program.
Someone I'm close to is considering buying a Switch, but is not too keen on having to pay for games that I already bought (both physical and digital). If the Family Membership plan will allow us to share games I've already purchased (and new games that he purchases), then he is much more likely to actually buy one.
If this person I'm close with gets his own Switch, can we get a family plan and share the eShop games we buy with each other, on our own devices? Or will he have to buy extra copies of games he wants to play?