Stunner911
He's joining at the perfect time because Anniversary events are pretty kind to newcomers with its events and freebies. He should grab the Anni Merry ship and then try farm as many gems as possible going through the story islands and the easier extra islands.
Story islands are a great source of F2P gems and then he can pull on the current anni sugofest (I think part 2 is best but part 1 is great too) as often as he can since 3x legend rate is amazing for a new account.
Kizuna Clash might be a bit too hard for a brand new account but if he can play some lowest difficulty and clear that, he can get some extra rewards. I do think it's better to focus on going through the story and clearing the easier extra islands.
He should take advantage of 5x skill up and 5x super success to power up his characters. No need to worry about limit break and that kind of stuff yet. As a new player, specials level and sockets are more important.
I'll assume he didn't reroll the very first Strawhat multi but in case he did and got blue V2G4/Snakeman, he's got the best newcomer captain.
Toadskii made this teambuilding guide for beginners. It's pretty helpful.
Your friend should never use a gem to revive if he dies, especially since there's 0 stamina until first clear now. If he can farm Coliseum Kidd and Magellan in Underground difficulty when they come back tomorrow I think, he should give it a try.
Also using a friend captain Kaido would help him clearing content he can't but at his low pirate level, he might not find many Kaido friends.
He should try farm the new (Ivankov and Marco) Fortnights in Extra island, at least on easiest difficulty, they drop turtles and berries and cola. Also he could get FN characters that could be helpful in the upcoming Treasure Map.
The game can be pretty overwhelming but he should focus on the easier content he can beat and try pull as much as he can on this anniversary sugofest since he needs units. :)