Thanks for all the responses. I know it seems very cut and dry, and to me it is, but a few members of the band want to give her an ultimatum before we make a decision. Basically see how she reacts when confronted and base our decision on that. I just don't think she will see what our problems are and will probably deflect and not take any responsibility.
She said she would sing the songs only at weddings but as a band we decided months ago so that we shouldn't have songs that we only play at certain gigs. We should have a song list of songs that are all great and that we could play anywhere. Respect crushes at any place we play. A club, casino, private event, wedding, dive bar, etc. She is trying to dictate what songs we do because she doesn't "like" singing certain songs. Also let's say it IS because her voice is giving her problems. Well she goes out on our off nights and goes to see other musician friends at local bars and gets up and sings songs with them and put unnecessary stress on her voice when she shouldn't. And like @Strobe said, RESPECT is not very high. She belts out other stuff like Whitney Houston and Journey but then can't do Respect? She doesn't do any type of vocal practice or voice exercises that I've ever seen. We even said hey let's just tune down 1/2 step and she got all bent out of shape about it. She said "It sounds weird". She doesn't understand keys or why we move the chords around to make it easier for her and for us.
NTAH up to your disregard for strained vocal cords. You could bash your guitar to bits and have a backup ready to play for <$500, don’t be a twat and dismiss potential bodily injury because you equate it to your guitar.
Edit: The only thing I think you are off base on is the singer saying she cannot do certain songs. Vocal chords are not like a normal instrument. They do not all have the same range. Some songs I cannot do at all. Some songs I cannot do without being very uncomfortable. I am a little surprised that RESPECT gave her trouble as it is not very high - but voices are kind of funny like this. You should find a singer who has it in their range - but as a bassist/guitarist/singer in various bands - pushing the singer to do a song that is uncomfortable for them to sing is kind of crappy. (P.S. You should still fire her)
She said she would sing the songs only at weddings but as a band we decided months ago so that we shouldn't have songs that we only play at certain gigs. We should have a song list of songs that are all great and that we could play anywhere. Respect crushes at any place we play. A club, casino, private event, wedding, dive bar, etc. She is trying to dictate what songs we do because she doesn't "like" singing certain songs. Also let's say it IS because her voice is giving her problems. Well she goes out on our off nights and goes to see other musician friends at local bars and gets up and sings songs with them and put unnecessary stress on her voice when she shouldn't. And like @Strobe said, RESPECT is not very high. She belts out other stuff like Whitney Houston and Journey but then can't do Respect? She doesn't do any type of vocal practice or voice exercises that I've ever seen. We even said hey let's just tune down 1/2 step and she got all bent out of shape about it. She said "It sounds weird". She doesn't understand keys or why we move the chords around to make it easier for her and for us.