Short answer: he shouldn't risk it.
The long answer would have a lot to do with WHY he's at risk of having a stroke. Is he prone to clotting? Or is it due to blood pressure risking an aneurysm? If the latter, the risk would be much higher.
I don't think you're going to find anyone anywhere who'll advise it's "safe" for your friend, unfortunately. Fact of the matter is, regardless of the reasons he's at risk of having a stroke, it'll be advised against.
All that being said, for what it's worth, I suffered a stroke (clot) just over 9 years ago (age 17 at the time). There was no medical cause found and was concluded purely bad luck (no risk of clotting, blood pressure fine, etc etc). I've rolled many a time over the past couple of years, and haven't had another one lol......buuuuuut that certainly does NOT mean it's safe. Even for me. My 2c.