I understand what you are saying. As a music director, ANYONE, catholic or not, who views participating in the music ministry as a means to perform is not in the right ministry. My music ministers, 99% of the time have the best attitude of service, so a “performer” does not fit in, and actually they quickly see it themselves and either adjust their attitude or leave. (by their own evaluation).
In many protestant services, they have more of a performance based music ministry, so the approach may be different, however, I know some and they have a heart for prayer in their music as well.
I have 2 non-Catholics in my Catholic lifeteen worship band. (Please no arguments about lifeteen here). My drummer has the heart of a servant, and he realizes that his job is to help the church sing better and drown him out, which they do. He plays so often for me for free and sacrifices a lot for the parish, including the music ministry. He has a way better attitude than many Catholic musicians I’ve worked with.
So what I’m saying is that just because of the situation, there might not be an “attitude” issue. THis may be a means of conversion.
She probably does sing in her new church, but the OP said she is home visiting and is singing at a Christmas Eve service. And while she may be proclaiming to be a non-*practicing *Catholic, she is still Catholic isn’t she? A more welcoming attitude may sway her back to her roots and return to the CC. Just a thought.
In the first place, you can’t judge her internal motivation for wanting to sing in the choir, that is between her and God. So you can knock that off your list of objections, I would think. What may SEEM to you to be performance oriented may not actually be. This is very subjective, depending on your opinion of they type of music you prefer, the way it’s played, the choice of songs, etc. What you see as a “performance” may be a sincere effort on their part to praise God. In the third place, God moves in mysterious ways, try not to get in His way. There is nothing wrong with her wanting to sing in the choir, and that desire might have been put in her heart by God. Enjoy the music and thank the Lord for it.