As far as I know there is NO grace period on the Plus loan. Interest will keep accruing.
If the first semester amount has been disbursed less than 4 months ago, as @kelsmom said, you can contact the loan servicer and your school and CANCEL $3500 of that Plus loan amount. Then any interest accrued on that amount, and the origination fee would be canceled as well.
Then only $500 plus interest would be owed.
If you don’t need the $4000 for the second semester, then don’t take it! Let the school know you don’t need it.
Your grandma is the one required to pay back the plus loan. So if it’s a hardship for her, then don’t have her borrow any more.