Praise be to Allah.
If this Shi’i believes in the tenets (‘aqeedah) of Shi’ism, such as regarding the Sahaabah (may Allaah be pleased with them) as kaafirs, hating Abu Bakr and ‘Umar (may Allaah be pleased with them) and other corrupt beliefs that they hold, then the truth should be explained to him. If he gives up (these false beliefs), then he can pray with you. If he does not give up those beliefs after you explain the truth to him, then he should not pray with you, and you two Sunnis should pray together. But if he thinks that he is a Shi’i (and he does not follow their kufr beliefs), and he loves Abu Bakr and ‘Umar, and thinks well of the Sahaabah (may Allaah be pleased with them), then he can pray with you and is not regarded as a kaafir.