Scizor/Cherrim is the deck for beginners, mostly since it's very much a hive mentality (no pun intended) sort of deck. You simply swarm, swarm, swarm.
Beedrill GE is easy to play once you get the hang of it. The problem is that beginner players will find it difficult to get all 4 Beedrills set up quickly, but once they understand what to do it picks up really quick.
Kingdra is pretty simple. You simply swarm Kingdras like you would with Scizor/Cherrim, but if you can KO with Aqua, do that. Tyranitar, on the other hand, is slightly more complex. You grind, yes, but sometimes it's more devastating to Spinning Tail the bench, and therefore it can be a bit overwhelming, especially the energy management in a format that's really more focused on speed. Gengar is also really easy to play. You simply snipe and flip, every now and then hitting big with Poltergeist, but can become both difficult and frustrating when your opponent closes in on you since it has no way to hit big for sure.
Standard Toolbox decks are generally fairly easy to play, and they're flexible to whatever you're playing. Although he won't get too far with a Standard Toolbox, it'll work around his playing style and can be molded accordingly. The more difficult decks, such as Legos and Dialga G/Palkia G, are more complex.
Don't try AMU as a beginner. It'll demoralise and frustrate both you and your opponent, as beginner AMU players tend to take forever to do anything and many times it leads to almost nowhere. Plus, it's difficult for them to manage the health of AMU when the strongest thing can barely attack.
I would suggest Kingdra, Scizor/Cherrim or Rampardos. All of them are straightforward, beat down decks that focus on getting set up fast and hit efficiently. Rampardos will require a bit more thinking because it self-destructs in the long run, so some management is required. Additionally, Abomazong is also very easy to play, set up and collect, as other than Cresselia X it doesn't require any super rare cards or cards that are in huge demand. Machamp is very easy too, and when you start to get used to setup speed and how they vary, Blastcatty becomes very nice.
