What is the ruling on theft committed by one friend against another friend's phone, and the complicity of the rest of the friends in not disclosing the incident?

You are not required to pay money to buy a new phone. However, you should have informed the phone's owner of who took it, provided that doing so would not cause harm, or advised the person who took it to return the phone or seek absolution, because his claim that he previously had a watch taken from him does not justify his taking the phone in this manner. If you can inform the phone's owner of who took it, then do so. And if you are asked to testify, you are obligated to do so if no one else testifies; for the right is established by a witness and the claimant's oath.