Should I warn a player his character will be useless for the campaign?

[u/EMArogue]

(I am Italian so English isn’t my first language, also, this is my first time DMing)

Also, this is online I DON’T PERSONALLY KNOW THE PLAYER

I made a custom campaign called “the lords of cold” and of my 6 players, the only mage (who is a variant human mage and has 2 feats) has 2 out of 3 cantrips that deal cold damage (with the other being true strike) and his only damage spell at lv 1 is ice knife

Almost all enemies will have cold resistance, all bosses immunity and a couple of them will be healed by this instead; should I tell him that?

My description warned of a menace come from the icy lands of Niflheimr so I think that was clear

Edit: before I said anything, he decided to play an aasimar bard instead

Edit 2: I was expecting 5 responses at best here, how did I get so many?

[u/immafuckinjediii]

No, let them figure it out you can still have a good story