I don't think you are in a position to help your colleague. What you are suggesting would be a complex task even without the colleague having Visa.
Not complex in the sense that asking raise is complex, but complex in the sense that you would affect 1. the behavior of your colleague 2. how your employer values your colleague 3. how your employer assigns raises.
Your message does not give enough context to verify you could "coach" him. Coaching generally requires the experience of successes and failures in similar situations. If this is an unfamiliar situation to you, then you are in no position to coach him.
It's well-meaning, but at the same time a bit condescending to think that you can assist him by some techniques picked up at an internet forum.
In addition, based on what you told, you don't know if he already has asked for a raise, and been denied.