Thanks for the replies everyone.
He's taken the Mage armor one and a couple of utility invocations. We looked through the list last night discussing it after the game and there really doesn't seem to be many other choices for him.
What are his other choices? Maybe you can provide us with his stats, level, pact, patron, invocations, most used spells, etc.
Mage Armour on its own is pretty weak. You can just have studded leather for 1 less AC. If you are a ranged character you don't need to max out your AC.
One suggestion the player made was to make the warlock spells recharge per encounter with a limit of say four times per day. What do you guys think of that?
Another thought I had was to increase the number of spell slots.
The equivalent of 4 short rests per day every day would make the Warlock too powerful.
Likewise increasing the spell slots would be too good. Though I could see giving the 3rd slot a level or two earlier. There is a lack of power gain until level 11.
The player is pretty disappointed in how the warlock is playing and is considering either multi-classing (something I'm not sure as DM I want) or dropping the character (again something I'd prefer not to happen).
It might just be that they don't like the style of the Warlock. Not all players want to play a Fighter or a Wizard either and that is okay.
The issue is that the party isn't getting lots of short rests.
How long has this been going on for?
The party should have an average of 2 short rests and 1 long rest per 6-8 encounters. Sometimes they should get less and sometimes they should get more to vary the challenges. If it has been going on for a long time then you may want to tweak something. Keep in mind that the adventure was written before the game was finalized.
So far in 5th edition the warlock is the first class design that's really made us stop and think that perhaps the designers got this class wrong.
My feeling is that it is group think.
Spells are a limited and valuable resource. They aren't meant to be able to be used every turn. They should be saved for just the right moment.
Eldritch Blast becomes quite good too after 5th level. It is not a waste of an action.
If the Warlock was able to cast encounter changing spells every turn they would outshine the rest of the characters.
Post the details of the character and the player's goals and maybe we can help to make them more effective.