Tarot reader’s often post a Code of Ethics on their websites. Generally they are very similar, saying things like “I’m not a doctor or a lawyer, so always seek professional advice. I keep my readings confidential. etc.” But they tend to vary on whither or not the reader will preform readings about relations, a staple of “fortunetelling” type readings. Some readers will do them, no strings attached. Other readers say they will not perform any reading that involves a third party without the third party’s permission.
When I do a relationship reading, I am (obviously) doing the reading for the person in front of me, but also for the person they have a relationship with. The cards are read with respect to their feelings about one another. In that way, I am not prying into the other person’s life, I think. If the querent was very tuned into their partner, they should be aware of their feelings. Of course, not all of us are that tuned in, so the reading just feels to fine tune.
Now, if the reading were, say, about “What secrets is my partner keeping from me?” then I would have to decline. That is going over the line, in my opinion.