You can get a 20% discount from a LBS. A quick google search shows the F5 lists for about $575. At a 20% discount it's about $465. Your buddy is only giving you $15 off but he isn't giving you the extra stuff you can get from a LBS such as:
- tubes
- tire levers
- bottle cage
- water bottle
- free adjustments for a year
- 20% off of other stuff such as a helmet, pump, etc.
Offer him $400 otherwise find a local LBS .
Also, do you know if the bike fits you?