I came across the following information and it really gives one "food for thought!"
COMMUNICATO IN SACRIS (Lat., joining in sacred acts). The act by which a Catholic joins publicly in divine worship with non-Catholics or excommunicated persons. Catholics may not participate actively in non-Catholic religious services, and anyone so doing becomes suspect of heresy; mere material presence may be tolerated on certain occasions…….
1941 Catholic Dictionary - Attwater
If an ordinary Catholic may not participate without suspect of heresy, what does that say of an archbishop who was an active participant?