In my observation, the iFly 700 screen doesn't noticeably degrade in brightness over time, but maybe that's possible depending on usage or other factors...I'm not sure.
Since you didn't mention it in your post, can you confirm that the display settings are all set to max brighttness? Specifically, Day/Night mode = "Day", and Brightness is set to max?
Also, can you confirm the problem is strictly the screen illumination, and not glare? The matte finish on the screen helps with glare but doesn't completely eliminate it, and in bright sun conditions glare can still cause visibility issues. Some users who have bright sun in the cockpit have had issues with white/light colored shirts adding to screen glare, and have seen better results when wearing darker shirts (which is no fun in the heat of the summer, so not always a great solution).
Others have had good results from fashioning a small sun shade around the device--think a strip of dark cardpaper about 2-3" wide and long enough to wrap around the top and part of the sides of the unit, sort of like a "cap with a bill and earflaps", to provide some shade to the screen.
I flew with a 700 for several years before upgrading to a 720. I never felt the need to use a sun shade, but I also fly a Cherokee so direct sunlight on the screen was not usually a big issue. I did notice a difference when I wore dark vs. light shirts, though.
Re: screen repair, if it turns out your screen definitely does not perform as well as your neighbor's identical model (you're sure he doesn't have a newer 720 or 740, which were both brighter than the original 700?), then yes, Adventure Pilot did have the ability to replace the screen. However, the 700 is their oldest model and I'm not sure they still have the parts to do that anymore. The cost is also a factor; I don't know what you paid for your used 700, but you might find screen replacement to be a poor cost/behefit option. A call to their customer support phone number during normal business hours will get answers to those questions.