Probably no big deal proving that he has passed wind and isn't getting stomach pains. it has been hot weather in the UK and he is probably just dehydrated. Tell him to drink plenty and monitor the situation for a couple of days.
your GP can prescibe Senokot and as friend is diabetic his meds are free.
Californian Prune juice [ Asda sells it] a glass every morning with breakfast helps my hubby + Senokot at night, he is on Metformin & insulin.
4 days "no go" isnt good for a diabetic, get him to the GP.