You can dispute the ticket on the DC DMV website. I've gotten a couple of DC parking tickets and a couple of red-light camera tickets from DC and VA, all for cars that weren't mine. In your friend's case it was probably some sort of database mismatch between the databases of DC and your friend's state DMV.
__________________
_________________
|...............| We live in the nick of times.
| Len 17, Wid 3 |
|_______________| [pics]
|