Best way is to ask him what the bumper numbers would be or to get the unit he was in. In some cases you need to know all the way down to the section level.
For example:
Passenger side front Drivers side front
28 1-110 IN A06
would equate to 28th Infantry Division, 1st Battalion, 110th Infantry, A Company, Vehicle 06 (which is usually the company commanders vehicle)
There is suppose to be set command policies on how to paint the numbers but PVT Joe Snuffy doesn't read the command policy memo's and can screw it up. Thats why you will see variations such as
28IN 1-110
28ID 1-110IN
28PAARNG (Pennsylvania Army National Guard)
1-110IN
As for the drivers side vehicle number:
At the line company level: 1 is 1st platoon, 2 is 2nd, 3 is 3rd, 4 is 4th, 5 is company XO, 6 is Commander, 7 is the first sergeant.
At Battalion and higher level: 1 is the S1(personnel), 2 is S2 (intell), 3 is S3 (Operations), 4 is S4 (Logistics), 5 is XO, 6 is Commander, 7 is CSM. Confusion reigns with all other numbers. Most sections keep the same initial digit. For instance the Bn commanders HMMWV maybe HQ-06 but he may have a tank or other armored vehicle for battlefield command that is number HQ-66.
Confusion deepens with headquarters companies and gets even worse when decoding support units especially above division level. Add to this the fact that certain types of units go away...such as the old E Companies that use to be the anti tank improved TOW companies the infantry battalions had in Europe or the old field bakery units.
That's why he probably needs to see if he can remember his old bumper number.
This is 3AD 54FSB C5 out.