If he's 18 and normally spends his time gaming and has only ever walked as far as Asda, his suggestion of going up Snowdon is a bit of a breakthrough moment!
Try to fan that flame, that he's shown an interest in getting out and about, it sounds like something that would really do him good, might even be life changing to discover there is a world out there beyond the screen.
That said, Snowdon for a first climb at this time of year for someone like him is just insane. And to get there on public transport is a complete arseache, believe me.
Can you not talk him/them into going somewhere more accessible for an excellent hike, and then go Snowdon in, say, May? There are so many beautiful and engrossing places to discover which carry less risk..
I reckon the inspiration to him and his mate has got to be the thought of Snowdon being a superlative. It's something to conquest/brag about/Instagram. Perhaps you could suggest a alternative destination walk eg the oldest bla bla, or the rarest bla bla.
Depends where you are based really.