What is the way to expiate the sin of helping to steal a bicycle, when one is unable to face the bicycle's owner or be open with him?

Theft is one of the major sins, and you have made a great mistake by assisting and covering for those who stole the bicycle. You must initiate repentance, and the completeness of your repentance involves returning the value of the bicycle to its owner if returning the bicycle itself is not possible; for merely abandoning the task of recovering a damaged item incurs liability. You are not required to disclose this to the bicycle owner; rather, you can return the value through any suitable means, and then seek reimbursement from your friends. Beware of associating with bad companions and seek out good company.