Make them a character that does a little bit of everything, ask them to roleplay (and make their own backstory, etc), then nudge them endlessly until they are equal parts RP and mechanics savvy.
It will be infuriating, but try to keep the annoyance on your side alone. D&D is rather complex so being annoyed AND not understanding things might convince them that they can't have fun so they just give up.
If necessary, get a shill to help boost their confidence and offer an enthusiastic partnership. The stooge can help play the new person up (so it seems they are 'winning' at their main focus) and take care of other things so you, as the DM, don't have to directly coddle them.