Is it permissible to fast on behalf of a deceased person, whether by his guardian or by someone else?
Machine translationRead the Arabic original
Is fasting specific to the guardian?
The relatives; the guardian is the relative. If someone else fasts, there is no harm, as a form of excellence in worship.
Q: Is it obligatory?
The Shaykh: What is known among the scholars is that it is recommended; because of His saying, Exalted and Majestic is He: [Al-An'am: 164]. So the closest view—may Allah know best—is that it is for emphasis and prescription.
The relatives; the guardian is the relative. If someone else fasts, there is no harm, as a form of excellence in worship.
Q: Is it obligatory?
The Shaykh: What is known among the scholars is that it is recommended; because of His saying, Exalted and Majestic is He:
and no bearer of burdens will bear the burden of another
وَلَا تَزِرُ وَازِرَةٌ وِزْرَ أُخْرَى
Source
binbaz.org.sa
The Arabic text is copied verbatim from the original source, without any edits.