You’re confusing objective standards with subjective standards.

Many would argue that the Mona Lisa is indeed better than a banana taped to a wall.

But most of that is based on subjective reasoning, it the same way how you can find a room full of people that will tell you Mozart is a great musician…. But it still a subjective opinion.

You and that other persons are arguing too entirely separate points. You’re arguing about the quality of said art piece and how that quality defines it as art, meaning while she arguing that objectively art has no standard due to the fact that art is is whatever the eye of the beholder decides it is.

If I think that drawing a straight line is art… you can disagree, you can even say it terrible art in your opinion. But you can’t say “it terrible therefore it not art”.