This paper addresses the optimization of the linear precoder of a relay shared between several source/destination links interfering one with the others, in the aim of reaching fairness between users. The sources are not aware of the relay existence, and the destinations behave as in a relay channel with interference. Thus, the destinations do not try to decode other sources' messages and the relay is the only enabler for user cooperation. We consider several figures of merit for each user, such as the channel capacity or the residual interference level after MMSE filtering, which are combined by a user fairness utility function.