I paid for someone’s medical expenses and now the persons grandparent are coming after me for the money they spent. Am I legally obligated to pay them

You are not liable to reimburse the grandparents just because you paid part of the expenses, but clearly there is more to the story.  Do the grandparentsclaim that you caused the need for the medical expenses in the first place, for example if the person slipped on a loose board in your house?  Do they claim that you are contractually obligated to pay the expenses, for example an employer may agree to cover certain expenses?  Are you the person's spouse or parent, who might be responsible for their expenses in some circumstances?  In these or other circumstances you could be responsible.