If your soprano is that soft, you might want to encourage her to take private vocal lessons, or even you give them to her to train her to sing the way you want her to sing. Also, I would really work toward recruiting more people into the mix. A second person singing with her on soprano may inspire her to take more vocal risks.
How do you rehearse new material?
That is very true, Formuzik.
I happen to be one of those folks who can't (or won't) sing soprano by myself. I'm just confident enough in my ability to hold down the part by myself. But, when I'm singing with another soprano, I'm much more likely to take risks and I reach notes I would never even attempt by myself.
(Just for the record, I'm not a real soprano...
or a real singer, for that matter... lol 
; I just do soprano when it's absolutely necessary).
This isn't a bad idea, at all. Asking your members what moves them can help move everyone.
Another thing you'll want to try to ascertain is what type (or style) of song really "moves" your congregation. I know, I know, we don't want to placate to emotionalism; but, one must start somewhere. Then, as they grow in God, you can introduce "higher" level music (although, I'd like someone to really explain to me exactly what "higher level" music is. Is it text that's supposed to move folks into a higher place, or deeper level in God?
). Oh well....
Keep us posted. 
Only problem I see with that is that it sounds like he's not dealing with real singers... and a lot of times non-singers can have some reeeeeally unrealistic suggestions for material. Often, it takes an experienced musician/director to be able to select material that is appropriate for their skill level...
Maybe if he gave them a list of appropriate songs to choose from?
I dunno. It
is a good idea, but it could take a bad turn.