A teammates on the line, not playing the current point is he considered player or non-player?
Rule 15.6 says that “Any opposing player may claim a violation” (out-of-bounds is considered a violation).
And the definitions define Player as “One of the up to fourteen (14) persons who are actually participating in the current point of play.”
So a teammate on the sideline cannot make an out-of-bounds call. However if a call is made, the players involved can ask the sideline for their advice.