When I initial installed Epocrates on the Droid Incredible, it didn't work correctly. After seeing some new comments in the android market, it appears that you have to have a microSD card installed for the application to install correctly. I did not have a card installed when I first put Epocrates on her phone.
When she gets home, I am going to uninstall the app and then reinstall the app since there is now a microSD card in her phone. I will report back if this did not get the program to work.
Update: Epocrates does work if a microSD card is in the phone during installation.