I don't know if you can just buy it over the counter but he would need to see a doctor before taking it and the doctor would arrange for him to get supplies
Getting to see a doctor is easy and cheap in Goa, you can phone for an appointment or just call in the local hospital
My wife felt unwell when we was in Anjuna and we went to the hospital near the petrol station, the doctor check her over and concluded she was suffering from dehydration in part caused by the mossie tablets she was taking along with her blood pressure tablets. He then insisted on check me over as well and he recommended that we both started taking re-hydration salts every day and used a different blood pressure tablet as the ones we use contained a diuretic. He asked us to go and see him the same time the next day and when we went back he check us over again and presented his bill of 50 Rps each