To get to the final answer a separate logic has to take place. Start by excluding yourself and your girlfriend and complete the shakes and you get
7,6,5,4,4,3,2,1. 9 shakes are missing from the total of
9,8,7,6,5,4,3,2,1,5
2,2,2,2,1,1,1,1,1,5 or 18 half shakes
since you know you are 5 that leaves 4 shakes for your girlfriend. This is probably the logic used by the riddler, but it is flawed and LiT is correct that you can not get it beyond 1-4, because the exclusions of the girlfriend from the 8 aren't possible exclude someone else and she has to be less than 4. The riddle is flawed.
P.S. It may have taken me a while and a few drawings of my own, but it can not be determined beyond 1-4. (without saying she went last.)
Breaking Stuff=Fun!,Bug Testing<>Fun!, Bug Testing=Breaking Stuff, so...
Bug Testing=Fun! Hmmmm....
DOES NOT COMPUTE! SYSTEM MALFUNTION!