the Mail even says
'She increased his blood sugar levels so tests would back her view that he had diabetes'
I suppose if you ate a high carb meal just before a supposed fasting test then the level might be high but you wouldn't be able to keep up the fiction for long.
perhaps it's significant that according to the Independent
'The 35-year-old mother was eventually caught out when she made a false rape claim to cover up why her son had missed his latest diabetes test. Doctors became suspicious and alerted the police who quickly discovered that, despite the raft of specialist equipment in her home, the son was completely healthy.'
It's incredible that she managed to fool everybody for so long though It seems that it might be a case where lots of individuals have suspicions but are afraid to voice them.