It's a noble sentiment, to be sure, but Nepeta is having absolutely none of it. It might work for him, but she doesn't want to leave him to the mercy of the desert and whatever he happens to come across next. Besides, she's fairly certain that at least one of her travelling companions would think less of her if she simply let him be. (It's less certain about the others, but still, she may as well do the most she can.)

"The least I can do is offur some of it to you as well. Maybe if you come back with me? That would make carrying it easier, too."

And less likely to leave tracks in the sand as she dragged the carcass back to where the others were too, which would probably help keep them alive for longer, if that was what Marius was looking to help with, by his offer of the elk.