Many people eligible for Medicare are also eligible for some Veterans medical benefits. If your friend is completely satisfied with the medical care and coverage he gets from the VA, there is no requirement that he enroll in Medicare, too. But most people find that at some point they would prefer to get certain care from outside the VA system, or that the care they want is simply not available from the VA where and when they want or need it. In those situations, being enrolled in Medicare would give your friend a much wider choice of health care providers. On the other hand, it means that he would have to pay a monthly premium for Medicare Part B medical coverage, unless he has very little income and few assets other than the home he lives in, in which case he might also be eligible for Medicaid, which would pay his Medicare premium for him.
If your friend does enroll in Medicare, he will usually have a choice about which system will cover a particular medical treatment or service. However, the VA and Medicare won't both pay for the same treatment -- it's one or the other.
Your friend might be able to receive a specific medical treatment directly from the VA system, which means the VA pays for it. This could include healthcare-related services that Medicare doesn't cover, such as physical exams and other preventive care, dental care, and long-term in-home and nursing home care. Also, VA co-payments are generally lower than Medicare's, including for prescription drugs.
Or for any particular treatment or service, your friend could receive it outside the VA system and have Medicare cover it (if he enrolls in Medicare, that is). If he's treated outside the VA system, the VA won't pay any part of the bills that Medicare doesn't pay.
To learn more details about how Medicare and VA medical benefits work in tandem, your friend can look at the Medicare online booklet Medicare and Other Health Benefits: Your Guide to Who Pays First.