Can a consumer rights case be made?

Hello sir, 

Around 1 month ago my relative gave his Yo-bike for servicing but now the service provider is saying that he has bought it from him and so can not return. Previously also servicing was done by same guy but nothing of this sort happened. As everything was done informally only no receipt or bill of service provider was taken. I wanted to know what would be best to do in this scenario.

Will consumer right case sustain without such document?

If there is any other way better than this then please do suggest it too.