Easy. Tell the lady that you will not charge her any money up front but you will build a guitar and there is no obligation, if she likes it when it is finished she has the first opportunity to purchase and tell her how much you will charge. If she likes the finished product, great, if not,
a) sell it
keep it
c) give it away
d) keep parts, give wood away, try again with new wood
or any other options referred to in PG,
but this way you should feel fair dinkum about the whole process and enjoy the build,
good luck with it.