Mary,
Here is my 2-cents. First of all - do you feel comfortable using 2 different references? Do you feel you can make the toes correct in the way she suggested? If any of these answers are no, or you are unsure - I'd speak to the woman as the professional you are - you know more than her on what it takes to make a sucessful portrait. Just tell her that piecing parts of photos together can have a funky feel to it and you want hers to come out as a masterpiece - in order to do that you need one reference that you both agree on - or go into particulars about why this one works over that one... Tell her to trust your artistic experiece...blah blah blah..
I do not quite understand which refence she wants for the head and which for the body - so I don't know how to comment on that in particular.... but I say just be sure of your reasons, use words like 'longevity in appreciating the design' or 'timelessness' etc.. And be slightly firm.
I suppose if it were me I'd start out with something like "You are the client of course and I want you to be happy. From my experience though - this particular one will be a better overall design and show your son in
such a wonderful light - it is really quite timeless.
Remember, this is an heirloom that will last generations, you want this to be a painting that is so well-designed that even his great-great-grandchildren, who never met him, will want it on their walls. Not only for the sentimentality issues, but because it is a great painting that stands on its own - even if you do not know your son."
How's that? Mother's! What a pain to talk to, eh?