I know families comprised of one parent Catholic and the other of different faith. Some of those families attend church services of both faiths.I meant, if she had already converted to one faith, she would feel she was betraying her own convictions if she attended a service of her former faith.
In my Catholic family, my high school aged son had a girlfriend who attended a nondenominational church. My son enjoyed attending church with her, however; our rule was that he had to attend mass with us before he attended church service with the girlfriend.