From the article

> Rob studied the man's face and vaguely remembered him as Ronnie Lockwood, someone he would occasionally see at Sunday School as a boy and who he was told to be kind to as he was a "bit different".

> Ronnie was then almost 30 and had been without a home from the age of 15, living in and around Cardiff and moving from job to job - Rob would sometimes see him at a youth club he ran.

> The pair planned to let him stay until the day after Christmas, but when the day came, they couldn't bring themselves to cast Ronnie out and sought advice from the authorities.

You aren't entirely wrong, but this wasn't a random person and they did contact a homeless centre for advice.

Given that Ronnie had apparently already gone through some sort of system to end up at a "school for subnormal boys", it seems pretty clear that Ronnie lived a much better life through this family's actions and generosity.