Pull them off and start over again with a more appropriate material? Not possible to do a job that doesn't look like ^&%$ without using the appropriate corner pieces. Having trouble envisioning what is trying to be accomplished here in the first place. I'd trim it out with azek and forget the fake stone.

Wrong material in the wrong hands. Has to come off, azec does seem a better choice

@rwiegand I explained that to her but she doesn't want to do that she said she just wants to fill in the gaps and make it look nice without having to redo it

Cover the corners with angled PVC trim.

I am not so sure, they can ever look nice, unless they are redone. Obviously the guy hadnt any idea how to do the stone, since it is a very poor job.

Tell auntie that there is no fix for that. Either live with it or do over with a more appropriate material.

Pre-formed corner pieces are EXPENSIVE. Which is why so many people don't want to pay for them. So, they push off mitering the stone corners onto the installer. But, they then have to hire the skilled installer. Who is expensive. Or, you get the silly butt corners, like was done here. Money was not spent on either material or labor here. You cannot make up for that critical lack. The cheap "solution" that fits that mentality is a big glob of ugly black caulk over backer rod. The RIGHT solution is to remove all that dreck and replace with Azek trim. The cheesy black stone is not at all complimentary to anything.

Sometimes a limb can't be saved and it gets amputated. That's what has to happen here.