I'm a regular platelet donor (every other Wednesday!) and working out before is fine. Just make sure you don't skip meals before your appointment and stay hydrated.
The problem with working out *after* is that you may experience dizziness or weakness, it's best to give your body a day to heal. You also wouldn't want to "spring a leak" - reopen your puncture site.
So work out early on Friday and enjoy it! Take it easy after. And don't forget the cookies and juice! That's why we all donate, right?