Purity: Will Stipulating a Specific Person for Salat al‑Mayyit
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 594:
If the deceased person had made a will stipulating that a particular person must perform the prayer for him, it is not necessary for that nominated person to get the consent of the guardian of the deceased, although it is better that he does.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)