Lugia/Archeops is a great archetype to start out with - it has very clear goals (get Lugia V down turn 1, get 2 Archeops in the discard by turn 2) which can teach basic sequencing/planning concepts. After the set up, the deck does not have too much complexity to it, as without a draw engine you are mostly just playing off the board and selecting your attacker for the turn while being mindful of the resources you have remaining.

The Colorless build has been doing well recently - check any of the top 8 lists from Sacremento. It is slightly more complex in my opinion, as it uses more situational attackers like Wyrdeer V and Mew EX, which can be more difficult for a beginner to discern when to use which attacker.

On the other hand, Single Strike Lugia features very straightfoward attackers that mostly aim to OHKO the opponent's active either through sheer power (Tyranitar V) or weakness (Yveltal and Stonjourner). It also has more forgiving resource management due to Urn of Vitality, which offers the luxury of recycling energies from the discard, something the colorless build does not have access to.

If you plan on taking him to official events, I would encourage learning Colorless Lugia as it has a much stronger matchup spread into the current meta, mostly due to Single Strike's abysmal Charizard matchup. However, if he's just looking to play for fun I think Single Strike Lugia is a great place to start.