That's pretty much what happens yes: you can grant him Advantage, meaning he can add +2 to one roll in his turn and keep it for future related rolls or spend it for +1d6.
And sure, roll 1d20+8: quick question for future ref, should I be rolling on PC's behalf in situations like this? I know different parties have different tastes: in this instance the game would move on quicker if I rolled it myself and made an IC update (i.e., we don't have to wait for me to reply to Kenny and for Kenny to get a chance to respond) but some have a preference for rolling every check themselves so that it's their luck, not the GM's