Ps3 – Possible to buy a game that you get free with Playtation Plus

Unfortunately, you can't do that directly off the PlayStation Network Store, since the buy option won't be available for games you own through PS+, only after your subscription has expired will that option be available again.

However, you may be able to purchase the full games through third party sources. For example, GameStop offers download codes for PSN games that can be redeemed on the PSN Store. I assume that if you redeem a code for a game you only "own" through PS+, you'll own it permanently (you can check the game's information in the XMB and see if it has an expiration date).

I cannot confirm this method 100% as I haven't tested it myself, so try at your own risk.

Not sure about the included membership, but PS+ Physical cards are codes to redeem in the PS Store. You may also purchase it via credit card on his account directly to have it applied to that account.