“the article states, Warren might have met Washington”
Actually, from what I see, the grandparents claimed he was “friends with” Washington, and the officer did not say he met him. That can be altogether different - a few remote letters, etc., which sometimes for some people was indeed as intimate as they became in those days.
In all that I have read on the General, I think his knowing Warren would have been mentioned, but I am at a loss.
Further, I could not find any evidence that Warren attended any Philadelphia meetings that Washington had, so I remain skeptical but welcome being corrected. The only other thing that comes to mind is Washington's 1773 trip to NYC to place his stepson in King's College (Columbia), so meeting Warren there is a possibility.
And yes, I am aware of the importance of letters during the colonial period, but one would be hard-pressed to call anyone a friend whom you'd never met.