My horse is also part Frieisan and has these feathers. Hers are black though. I'd leave them because you're friends are right, it's part of who he is. It shows other people that he has a different breed in him and that he really isn't just a severely oversized paint. You'll just have to clean him more often if you have the desire to keep those clean.
"And somewhere in the northwoods darkness a creature walks upright. And the best advice you may ever get is: Don't go out at night..."