It happens often that when a musician upgrades his or her knowledge and competence in music, the liturgy committee will suddenly emerge to "express its concerns" about the growing "conservatism" or "traditionalism" of parish music, and call for more peppy hymns. They might even commission a parish poll on what people want.
This sort of thing makes musicians crazy because it is a setting guaranteed to yield shabby liturgy and community chaos. It is the worst possible thing to happen to a parish music program, and not because the community shouldn't have a voice. If the community has a point of unity, it concerns the faith itself and the tradition; otherwise, in terms of issues of taste and preference, there is no such thing as a community: there are only individuals with a multiplicity of conflicting desires. A method of liturgical planning that exalts the "desires of the community" over the demands of the universal Church yields a very divided parish, with egos clashing against other egos, and to heck with what the liturgy is calling for.
It would be the same if we chose the texts or the vestments of the Mass with this method. Nothing good can come of it.
The most important thing for a musician to do in these cases in to remain calm and remember that those intervening are in deep need of catechesis. There is not much time to do this, since seminars are not exactly the way people want to go. What a parish like this needs is for the liturgy committee to quickly face a different reality.
Documents such as the GIRM are good, but they can be confusing. What these people lack is an understanding that the music of the Mass is a given. It is an embedded part of our tradition. I would suggest that the musician in question quickly get a copy of The Gregorian Missal, which has all the Sunday propers and the ordinary for Mass in Gregorian notation with English translations. No book better illustrates the point that the music of the Mass is part of the structure of the Mass, not something that is chosen by a committee or by democratic methods.
Let everyone pass this book around, so that they can see for themselves what the music of the Roman Rite is in fact. This is the core repertoire, what the Church is asking us to sing and has asked us to sing since the earliest years of the Church. Gregorian chant was exalted at the Second Vatican Council as the music of the Roman Rite because it is holy, beautiful, and universal. The musician can explain that this is the liturgical ideal and that everything else that we sing or do is, in fact, a substitute for this ideal.
Once that is understood, everything changes. People begin to see that it is not about the community's needs or the musician's training or preferences. It is about the universal faith. The standards are different. Now, you can't do all propers and ordinary in your Mass; it is not practical in most cases (though there are plenty of parishes that have achived this). But chant remains the standard by which all substitutes should be judged -- a point made by every Pope dating back a thousand years, even back to early Church.
If the musician can get people to see this, and the pastor too, the entire environment will change. It is absolutely urgent that musicians do what they can to help people understand this point. Any other path can lead to disaster.