Trees already on a Muslim's grave: what can I do, are there any rules?

Allahu Akhbar. Bismillah Arrahmani Arrahim. Peace and blessings of Islam to everyone. Before I became a Muslim, I planted two trees on top of my Muslim friend’s grave. They are growing well. After two and a half years now, the mother of my friend has objected to the trees and wants them removed. They are 2m tall already. I became a Muslim one and a half years ago. I am now aware of the ruling that it is Makrooh (Makrooh: it is not forbidden, but it's better not to do) to cut trees in graveyards. What should I do about this situation? Can the trees be cut down without violating any Islamic rule?