Sorry to have to tell you that a Medical Card is not an automatic entitlement for all Invalidity pensioners. Means testing does come into the equation. If he has no other monies or income sources he would probably be eligible for a medical card based on the Invalidity pension rate alone, but such matters as to whether or not your friend is living alone or with dependents or indeed is financially dependent on and being supported financially by another member of the household etc can be an influence in the Department of Health's decision. However, if your friend has a lot of medical bills, i.e. for doctors visits, chemist bills etc. which are likely to be ongoing he could apply for a medical card in his own right (for himself alone as opposed to a family medical card). In addition to completing the standard medical card application form he should supply a letter from his local G.P. and hospital doctor/s supporting his claim. This is quite a common practice and his doctors etc. won't be suprised if and when he asks for such a letter/s.
Hope things work out for him