Regarding entering a Reform or Conservative Church, see Igros Moshe O.C. 2:40 and Igros Moshe O.C. 3:25.
Regarding tznius, it should be made absolutely clear that a frum or non frum relative or guest (of either a FFB or BT) is not welcome at a Simcha unless completely dressed per Hilchos Tznius.
Regarding attending an event of a non-frum person, if there is mixed dancing, amongst other issurim (i.e. tznius), it would be prohibited.