I have met a Jew from the Ucraine who has joined a Messianic sect called Beth Sar Shalom. He`s a humble and warmhearted person truly searching for G`d and feels very close to Israel in her time of war but like many Jews from the former UDSSR he`s religiosly uneducated and understands no Hebrew. Now he has shown me some of the proof texts they had shown him and naturally my Scriptures are saying often something different than his Christian ones. Should I prove these "proof texts" to be wrong and become active in this with the danger that this person might lose his faith in G`d at all or should I leave the whole matter to G`d to be settled without me doing anything?

It is the obligation of a Jew to teach others Torah and save them from sin. Although this obligation has many facets, the obligation to save another Jew from apostacy is great. If you think you can be of some influence, and are capable of refuting all or even many of those so called truths, I think you should try to do so with much love and concern.