The party I'm running for is a tiefling, hill dwarf, kobold, and minotaur (half-orc reskin), so they all have darkvision. It's a different experience.
Huh. My group actually runs with darkvision races so much that often the non-darkvision races usually end up with Goggles of Night to make up for it.
I played a Triton GOO Blade Pact warlock for a bit. The main thing is you're squishy, but having a barbarian and a cleric should help him out there.
I used Armor of Agathys a lot (temp HP and cold damage when hit in melee is great, especially since it scales up), as well as crowd control spells like Darkness, Hold Person, and Hunger of Hadar (GOO also get Dissonant Whispers) to protect me from being hit. I tried using Hex and it was helpful, but I got hit often and lost concentration too many times to get maximum usage out of it.
I also recommend seriously considering starting or MCing at least one level into Fighter as well. Second Wind, Medium or heavy armor, and Defense will help a lot. (If he ends up not like blade pact warlock but wants something similar, he might like Eldritch Knight or Valor Bard).
The time I played a blade pact warlock before Hexblade was released, I played a half-elf Feylock. It is nice when you get to 6th level and can just teleport away and turn invisible when you feel like you've taken too much damage. I also started with a level of fighter so I could have heavy armor, which is good if you want to fight in melee and don't want to worry about having to escape eventually (though I could if I needed to). I mostly fought in melee with the character and saved spell slots for utility use, like turning people invisible or casting Seeming on them.
Outside the topic, but related to spellcasters who fight in melee, lately someone in my group has been playing a Bladesinger,, and I have to say, it's interesting to see a wizard being played as a melee fighter. He apparently has less HP than my Conjuration wizard, but he gets away with it for the most part from having a high AC, especially when he casts Haste on himself (which is most of the time), though I think he has to use Shield more often than I do, since I don't get hit as often, and blows are more likely to be above what I can block than they are for him. From what I've seen, it seems like the archetype strikes a nice balance of doing what it's intended to do without being overpowered, which is nice.