For a believing Catholic, the reception of "communion" in say a Protestant Church , receiving bread and wine, would constitute a denial in the dogma of the Real Presence and would be a sin against the First Commandment. Objectively speaking that is.
How can you objectively know what the person is believing? Objectively speaking, that is. If they think they’re getting the real thing, that’s a problem, but if they’re just sharing in the nibbles with the rest of the folks, where’s the harm?