I think the best strategy is to ask if anyone knows the words. Usually someone does.

I have been at several sessions/singarounds where being able to help out with the few forgotten words has saved both a song performance and a singer's self-esteem.

And it has happened to me in the past too - no-one can say they are going to be word-perfect. I always appreciate someone giving me a clue as to what the next few words are, 'cos that's all it takes to get back on line.

My opinion on singing from books is best kept to myself.

I do, however think that making an example of someone who has trouble with learning words is not a very kind way to encourage them into what is a very dwindling interest group. Why can't we be generous and welcoming?