Crack it open and post another photo, why? The cortex or outside skin looks worn, it betrays the actual mineral/rock type, breaking it open or chipping a fragment off gives you a view of the raw inside.
Well, do a hardness and streaktest. Or have an assay done, to be really really sure. If I'd expect silver, for what ever reason, I'd be inclined to do so.
How did you brother come to believe its silver ore? Did he collect it from a known mine?
Difficult to say what it is, and ore yes, but having collected material like that in Colorado as a kid I'm inclined to think its more likely to be pyrites, galena or a combination there of as they have similar appearance and are much more abundant in that locality.