A friend of mine has had Type 1 diabetes most of his life, and has it well under control. How does he go about finding out if he can get his medical and learn to fly?
As far as the FAA is concerned your friend can get a medical, but he will need a special issuance medical, will only qualify for a third-class medical, and will be prohibited from operating aircraft outside the United States.
In addition there are some specific requirements to obtain the special-issuance medical for insulin-dependent diabetics, and monitoring requirements before and during flight which must be adhered to.
The FAA has a page of guidance for Aviation Medical Examiners that deals with insulin-dependent diabetes which your friend should review (and be prepared to discuss with their AME).
Diabetes.org also has a page on Diabetes and FAA Certification, and AOPA's medical folks would be a useful resource to help ensure that the special issuance medical goes through without any hiccups.
The main problem in Type-1 DM is that anything that increases the body Glucose consumption may yield a potentially obtunding and sometimes deadly too low blood Glucose levels (Hypoglycemia in medical terms), this may come from a physical exercise while on the usual Insulin dose, and also from the mental stress from piloting, that may increase the Glucose consumption and lead to the catastrophic event of a loss of conscience, a blackout, or death while in command of an airplane. Automated Glucose monitoring devices that act mimicking the Pancreas islet cells function, measuring in a continued real time blood glucose levels, and releasing Insulin to the blood in accordance, are under strong research by many, but as in both T-I and T-II DM, (DM may be considered a mainly self-managed disease), the race is against the long term complications from poor, too high or too unsteady blood Glucose levels control, a too low blood Glucose level is an immediate life-threatening situation. People suffering T1DM must acknowledge that some activities are too dangerous for them and in certain circumstances for those close to them, at least in the current state of the medical arts, and thus, T1DM patients better refrain from engaging in some types of risky activities. Acting as an Airplane pilot is one of it.