My own personal advice would be for all Protestant hymns to be forbidden during the Divine Liturgy.
However, if people like them and were raised with them and miss them terribly, then let them sing them at home. There is nothing wrong with that.
In my parish, for instance, we never sing non-Orthodox Christmas hymns during the Divine Liturgy. However, BEFORE in Christmas Eve Vigil, we gather in the Parish Hall for the Holy Supper. And after the Holy Supper, we sing lots of the Western Christmas hymns, both the Protestant and Catholic ones. Sometimes kids in the parish that play a musical instrument will entertain us too. But it all stays in the parish hall. It never intrudes into the Divine Liturgy.