Stupid question, perhaps, when you say she is "given" tunes, who is giving them to her? If the director of the group is handing her music that she feels is out of her range, she can't just change the key to suit herself--it is the director who picks the key--

If the director chooses to change the key, it is a simple matter to simply start on a higher or lower pitch--the written music stays the same when you are singing (though the accompanist may have to do a bit of juggling)--