Hike negotiation

One of my close coworker rather a best friend, recently talked to his manager for promotion. However the hike after promo what manager said was ~25%. However same experience, same team -same experience guy who got promo in last December cycle got fixed around 28-30L. Now he had asked suggestions. He's fixed is coming around 24. He asked me how to get fairly treated. Both are having same experience, same team. However the earlier guy joined at higher fixed. Is there any way to see this as he can't compare himself directly. I am not expecting promotion this year but this seems to be a genuine question to be treated fair. If any such scenario comes to me , I would simply get an offer and leave. However I want to know the correct approach.