It really depends on the severity of the diabetes - do they control it with diet? tablets? or injections? I would have to say it depends on their age too and how long they've had diabetes for. Then make an informed decision as to how to treat them.
I would say there is no problem with regards to Indian Head Massage as long as they have healthy and responsive skin with good circulation. As for Reflexology, be aware that people with diabetes can have loss of/reduced sensation in their feet - ask if this is a problem for them and always check that the pressure is ok. If they do suffer with this then ligter pressure will be required.
The longer someone has had diabetes for, the more chance that they will have reduced sensations so if they have recently been diagnosed - there shouldn't be a problem with this. According to A Massage Therpists's guide to Pathology, "Neuropathy generally appears about 10 to 20 years after diabetes is diagnosed. About 30% of diabetes patients have some degree of neuropathy." The book also adds the following with regards to Reflexology treatments: "Work as part of health care team: focus on pancreas, solar plexus, liver, gallbladder, spleen, lymphatic system, kindeys, intestines points."
Check with their GP if you like but it is unlikely that you will receive consent from them - the doctor won't sign their name to say they give consent especially to something they don't understand and are not a specialist in. You could ask your client to mention to their GP that they are having the treatment - they would probably say to themthat it would be very beneficial to them.
If they use injections, avoid the injection site during massage.
I would say that if they are healthy with no complications associated with their diabetes then go ahead with the treatment. If you're not sure, perhaps do 30 minutes of each to start and then build it up once you know how they react to the treatment.
Mention to them that after a treatment their blood sugar levels can drop so advise that they check their levels before and after the session.