Pitch pipes only help those who can sing in tune, as Eileen can, to find where to start. Even then, that's not foolproof as I've seen some disasters where people have confused start note of the song with key: not all songs start on the tonic! I have suggested, as
Jack says, that singers might write down their preferred key on their crib sheet, to which came the heated answer, "I know nothing about keys!", to which I would gently say, "but you're singing it in A and that suits your voice, so next time you sing it, ask for a musician to give you an A".
Pitch pipes don't help those who wander around keys (why do they always choose desperately chromatic songs?) An accompanying instrument can sometimes help people stay on key: I did just this very quietly for a friend who had thought she would sing a capella but then wanted a wee instrumental between verses: on her own, she drifted upwards, but with me doing a very restrained basic chordal accompaniment, she stayed on key.
And I've long since stopped carting the big song folders (and tune books) to sessions. Just a couple of sheets plus a few in the head. If the one you really wanted to sing/play has been left at home, tough, you can always do it next time!