Wirelessly posted (Samsung Galaxy S: Mozilla/5.0 (Linux; U; Android 2.3.6; en-us; SGH-T769 Build/GINGERBREAD) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1)

At the very end of the page (link provided by A2A), it's stated that baggage allowance for rest of the world is 20kg ij economy class.
Since DEL- LHR-DEL is not mentioned explicitly under any of the prior headings, I would presume that ROW rules apply.
The 46kg allowance is for LHR-DEL-LHR. The OP made the wrong assumption. Hed he called to confirm his assumption, he would have found out that it was wrong before his mother left for the airport. It sucks tjat different rules apply for trips to LHR originating in DEL.