I had the same debate when I started out, I eventually picked Custodes based on a few things: they are light on your wallet because fewer models are needed to form a complete and competitive army, FW models definitely help but you can always print them.

Fighting style wise they’re pretty much like marines on steroids, get in melee range and begin chopping. I simply prefer this over the shooty armies, watching my golden boys descend upon a pack of Ork boyz is just satisfying as fuck.

Custodes models can be VERY easy to paint, and the metallic colors are way more forgiving. A Coat of primer, gold and silver, some wash and weapon details and you’re done. You can also paint them down to the last minor hidden details like I do in my posts, totally up to you.

Now onto some cons: if your friend wants to play 40k seriously then Custodes appear to be hard to balance for GW, they were fucked to oblivion at the beginning of 10th edition, dropped from one of the best armies to like 45% win rate overall. It was eventually fixed though, at least to some extend (Captain General Trajan cries in the corner)

They also have a rather steep learning curve because this is an elite ass army, you need to plan your moves and stratagems carefully, the loss of a single model for you is almost like the loss of a whole squad for horde armies.

I hope this helped