Diabetic here (on Insulin).
It's all about planning and making sure you are well fuelled before you start. Not sure what they mean about not feeling quite 'right', but assume that this is possibly getting a bit low (hypo). I can tell when I'm getting low as I go slightly 'wobbly', get even more stroppy and have a slightly 'spaced out' empty feeling. When this happens, it's all about getting a good hit of glucose quickly, so I have energy tablets with me all the time (lucozade, etc) so I can get a good quick hit to get me going. Otherwise it's making sure that there is a good supply of slow release sugar.
As your mate isn't on insluin, they shouldn't reallty suffer from hypo's as this is the effect of the insulin and a reduced sugar intake.
Everyone is different, so they need to do a bit of experimenting, and it's worth them doing a quick test when they feel not quiet right to see if's a hypo (low) or hyper(high) issue and then they can start adjusting to what works best for them