So my singer wants to sing and play bass. She plays piano fluently (she's been playing since she was 4), knows a great deal of theory, and our bass parts aren't that complicated. Plus she's the type to obsess over something until it sounds right. She's 20. Does this sound like a foolish endeavor or does it have a shot at working?
sounds like you could learn a thing or two. i'd say it has MORE of a shot at working.

what's your problem here? i don't really see how this affects you.
Anfangen ist leicht, Beharren eine Kunst.
She's the singer of the band I'm in. I'm giving her my bass and I think it'll work I just want the blessing of an internet forum.
It can be done. I did it a few years ago, in the reverse order though, from bass to piano. Granted I'm a little younger and arguably at the time can learn faster, but having theory is a huge advantage which I had to pick up as I went along. I would suggest giving a fair amount of time for her to get the full feel for bass before giging/recording/major rehearsal whatever.
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does she play piano for the band?

if not, you lose nothing. just make sure her technique is fair at minimum and you're set. theory does you a world of good (possibly more), but if you can't play the bass, then what the hell good is it?
Anfangen ist leicht, Beharren eine Kunst.
She only plays piano in two parts, so we could re work the arrangement or just use piano as a bass instrument for those parts.
could totoally work. and she doesn't sound like the kind of person whos gonna go up and try it on stage if she doesn't think she can pull it off.
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I think bass is a very easy second instrument to pick up (this isn't to say "bass is only a four stringed guitar" btw).

Bass was my second instrument, and yeah sure, while she'll need to work on the technique a bit (as my first instrument was guitar I could adapt a bit easier than a piano or horn player would be able to) but as she knows theory and has played music for a long time she should have no problem playing the instrument once the technique is down.

Definitely not a foolish move.
I switched from piano to bass. I had relatively good theory knowledge too so once i had good playing technique down then I found it easy to pick things up. One issue you may have is her co-ordination initially, obviously the bass will feel foreign to her in the beginning and she will need time to learn how to play and sing together effectively.

So long as your cool to deal with a couple of potential 'teething' problems then its a great idea
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