This depends on how tall your partner is, as a 54 fram is a bit on the small side for a taller rider, I am 6"1 and need a 56 or 58 depending on the make of bike. I have a TT bike that is a 54 but this is all about speed and getting compact so a smaller frame is the norm.

Assuming that is the Carbon frame, then it sounds like a great price with two caveats:
1. Be absolutely sure that the carbon isn't cracked
2. The Cannondale Six 3 is going to be a very stiff frame with an aggressive racing position

I've never used SRAM Force but I am told that SRAM is usually either a love/hate relationship especially with the right shifter.

I've had a set of Kysrium Equippe rims that were given to me. They handled my weight well until after 5,000 miles the spokes developed fractures stress cracks in the rim rendering it unusable. I don't know how many miles were on the rims previously so this could have been well within the life-cycle expectations and the rims did not fail even though I did some 50mph descents the weekend before the cracks were discovered.

If he can ride the bike and likes the ride, then I'd buy.

If he is 5'8" with shorter legs then the 54 might work. The price is good (for a bike without frame damage) but a waste if the bike doesn't fit. Best thing to do would be for him to test it on a trainer and get it dialed in and then see how it feels (even better to have a bike shop be in on the fitting). To set up the seat in relation to the pedals I use a plumb line from the knee cap running a string down to hit the middle of the spindle. As far as seat height goes, I try to make sure that the leg is not able to lock out straight unless the heel is dropped, just a touch of leg bend with the shoe flat at full extension.