Upvote:1
I believe your mother should write an e-mail to the consulate of one of the Schengen countries located in Sri Lanka, where your mother should apply for a visa if she would not leave to the UK, describe them the situation in detail and ask for the permission to apply for a Schengen visa from the UK.
I, a citizen of the Russian Federation, had the same situation when applied for a Schengen visa (Germany) from Brazil where I was as a tourist together with my husband who had an internship in Brazil. We were in Brazil for quite a while, so I was not able to apply for a Schengen visa from Russia. I wrote to our local German consulate, described the situation, asked for permission to apply for a visa from Brazil, got it, applied for a visa from Brazil, and successfully got a visa.