When you see painted guns in films like Lone Survivor, or on SEAL Team, or even real life like Obi Wan Nairobi/Chris Craighead, they do look cool AF, especially when they're 'worn in'. Sadly, the majority of painted guns I've seen on airsoft sites (or in the classifieds recently ?) look utter dogshit.
Your mate's however looks pretty good, but I'd always have in the back of my mind that a paint job is very personal. As others have said, if you want to sell it, your're knocking a few quid off the asking price. But how can you put a price on your enjoyment?
In conclusion, do what you feel man and enjoy it. And at the end of the day, it's an MP7, so automatically you're cooler than at least 68% of other players on the field!