If they keep Shabbos and Kosher (really keep, with all of the Halachos) I wouldn’t call them Mumrim Lehachis or say that if they touch your wine it’s Stam Yeinam.
To stick my two cents in, to whom do they ask shailos? A conservative temple leader, or a Rov? Also going to such a service in of itself is an issue. Would you say the same about a Jew who just happens to go to a Unitarian church?