It may be worth sitting near where the bicycle is locked up for a day or two, seeing if it gets used, identifying the person who uses it (If it ever gets used at all), and figuring out where they live. Try to get photographic evidence of the person locking/unlocking the thing. Then you can either.

a.) steal the bike back

b.) follow the person to their domicile and make a scene or violent scene, questioning them and perhaps, if the person admits that they stole the bicycle, exacting some form of physical violence and/or theft representing the amount of physical labor you put into purchasing and building said bicycle, the value of which has since been removed from said bicycle by its mistreatment.

c.) call the police report the location of the bike and description of the current "owner". be ready to present your receipt, have your friend's shop phone number, email, and address to corroborate your receipt, have pictures of your girlfriend on the bike in the past, and any other photographs, and try to have photographic evidence of the person touching/locking/unlocking the bicycle