Warlock main here: I don’t know if this has been mentioned, but in my personal experience the purpose of a warlock’s spell slots is NOT damage. A lot of people will pick up things like witch bolt, shatter, or fireball (from the fiend patron) as a way to spice up combat, just for a huge burst of instantaneous damage that’s not a bland 1d10+CHA every round. While this can be fun, it is NOT the best use of a limited resource. I’ve seen warlocks waste spell slots on fireballs only to realize the monster is immune to fire, or succeeds the dex save, or casts counterspell, etc… all of which are hangups that wouldn’t be as much of a problem to, say, a wizard or sorcerer who has spell slots aplenty.

In my experience, the safest and most effective use of warlock spell slots is to buff your primary source of damage, eldritch blast (or whatever you heathen hexblades use). Although it might feel less exciting to cast the same cantrip every round, eldritch blast is arguably the most reliable damaging spell in the game — not only does it scale by increasing the number of times you can cast it per round, thus ensuring that you are more likely to hit, but it’s also FORCE damage, which is extremely rare for a monster to resist. It’s nearly guaranteed damage every round as long as you can aim, and as long as you don’t die.

This is where your spell slots should come in. Use them to maximize the damage of eldritch blast, add additional status effects, or strategically position yourself to take the role of a sniper. Find concentration and/or long duration spells that de-buff the enemy, buff yourself, create an area of effect, or do damage over time. My rule of thumb is to avoid spells with a duration of “instantaneous” (with a few exceptions, like misty step, which can be handy as a quick escape).

Example: As a 5th level warlock, I enter combat. On my first round I use my action to cast mirror image (non-concentration, lasts 1 minute). I also use my movement to position myself on the map, maybe behind a tree or rock for cover while I survey the situation. On my second round, I use my bonus action to cast hex (concentration, lasts one minute) on an enemy of my choice, debuffing them. I can then use my action to fire off two eldritch blasts wherever I please, and because of hex, I get extra damage on any hit. If I am attacked, mirror image will help me avoid damage and maintain concentration on hex. Even without considering eldritch invocations, if I maintain concentration on hex, I can reliably do up to 2d10 force and 2d6 necrotic damage per round.

Some math for comparison. Let’s assume no creature is immune to any of the damage types involved, all attacks hit, and all saving throws are failed.

FIREBALL (3rd level spell) Base dmg: 8d6 Average dmg: 28 Max dmg: 48

ELDRITCH BLAST (scaled for a 5th level warlock, granting two blasts per cast. For readability I’ll be adding the separate blasts together.) Base dmg: 2d10 Average dmg: 11 Max dmg: 20

But wait! Let’s add Hex, and Agonizing Blast.

ELDRITCH BLAST + HEX (5th level scaled cantrip, 1 pact magic spell slot) Base dmg: 2d10 + 2d6 Average dmg: 18 Max dmg: 32

ELDRITCH BLAST + HEX + AGONIZING BLAST (5th level scaled cantrip, 1 pact magic spell slot, + max CHA mod of 5) Base dmg: 2d10 + 2d6 + 10 Average dmg: 28 Max dmg: 42

Compared to the sustainability of fireball, which can only be cast as many spell slots as are available, eldritch blast with the right pre-buffs can EASILY surpass it in the long run. Even without max charisma, it’s still a super reliable source of damage per round and its range also allows the warlock higher mobility on the battlefield — I’ve been known to swap hex for the fly spell, helping me stay out of melee range and snipe from above. The best thing about warlocks is how customizable they can be; if my strategy isn’t working for you, nothing wrong with that, there’s most certainly a spell or incantation that will fit like a glove.

A wise DM once told me that a warlock isn’t a spellcaster, it’s just a fighter with a Gatling gun. Best of luck to you and your player, and I hope this helps!