DnD character help?

My DM and I was making a new character for my GF to play (she makes lousy decisions) and we realised that the Warlock is being short changed compared to the Fighter when starting at level 17 since the class feature of replacing spells each level is meaningless when you can just go straight to the spells you want.

Seeing as all classes are supposedly balanced, a 17 Warlock is going to br behind tge curve of a 17 Fighter: What you get is less than you pay for!

Should we give the Warlock a feat every level to compensate for this, I don't know if that will be enough. If we can't solve this we may have to give my GF an actual Fighter instead, and I don't know if I can handle the STR difference (I play a were-Tortle Monk/Bard)