Rav Zilbershtein said that he told the family they are not allowed to sit shiva, as telling the grandmother or giving her any reason to believe her grandson died in Meron would be dangerous for her He said they cannot do anything different - they should dress regular, not sit shiva, not be sad, etc. They can tell her that he got hurt and injured, but that they had to send him abroad for treatment and surgery so she wont be able to visit him or talk to him, etc.

And they should have someone near the grandmother to ensure that nobody will come in to visit her and inadvertently let it slip, maybe offering comforting words to her, etc.

source: Hamechadesh video