I commend the sentiment but doubt saying "btw, why don't you give x a promotion too" is the right way to go about it. First of all, your colleague should fight his own battles and if this promotion doesn't put you in an awkward position with him then why is it your concern? Second, if something did put me in an awkward position with a colleague I would talk to my colleague directly rather than to my boss about my colleague. Maybe your colleague doesn't want the promotion (added responsibility, not enough pay to justify, etc). If you want the step up I would 乖 乖 accept the promotion and then raise your issues after. A bird in the hand and all that.