Different reflection colors do not automatically mean one lens is fake or defective. Lens coatings can appear different in color because of small differences in coating formulation or thickness, and manufacturers sometimes revise coatings during a product’s production life. That said, the serial numbers you gave suggest your friend’s lens was made earlier, so a factory coating change is possible but not certain.

Normal sunlight use does not typically “wear out” modern coatings. They are not supposed to fade just from outdoor shooting. A more likely explanation, especially for a used lens, is that the front coating was altered by previous cleaning, abrasion, or chemicals. Damage or partial removal of coating can change the reflected color.

So the most likely possibilities are:

1. a coating variation between production runs, or
2. coating alteration from cleaning or wear on the used lens.

By itself, a color difference in reflections is not strong evidence of counterfeiting. What matters more is whether image quality, contrast, flare resistance, and overall finish match expectations.