Elderly grandmother fell and swears she heard her skull crack but she refuses to go to the er, what to do?

Neuro checks: Make sure she understands consequence of not going to ER. Assuming that she's otherwise ok, make sure that nothing else is injured. In other words, that she's breathing fine (no broken ribs), walking fine (no broken hip), etc. Then monitor closely for any change in mental status. Unfortunately, subdural hematoma can take weeks to make itself known while epidural bleed can quickly be devastating.