Cresselia is a good TR user in non-TR teams (TR=Trick Room). I like to play with Gothitelle in TR-teams, as you can trap you opponent and low their offensive stats while seting up yourself. There's a good core called "GothLax" which is Gothitelle + Snorlax. You can pair it with Mega Manectric and Persian, Intimidate + Snarl on Manectric and Parting Shot on Persian. You can use Volt Switch or Parting Shot and send out Gothitelle, which traps you opponent's Pokémon, so they have to pray for a crit. Then, you send out Snorlax and start setting up with either Belly Drum or Curse. You should use 50% berries on both Gothitelle and Snorlax, and use Recycle on both of them. Also, Gothitelle can use Heal Pulse to heal Snorlax.
Another option is Mimikyu, which was very popular last year, in a core called MimiLax, but this year doesn't work as last year. Mimikyu can either go for being a support Pokémon or a sweeper itself. It usually runs Trick Room + Shadow Claw, and then you can go for Z-Destiny Bond, which acts like Follow Me and use Belly Drum on Snorlax.
However, I think that you friend is going to be very limited to TR teams or rain teams, as other teams usually use Landorus and one Tapu, which is a bit of a pity :(