Princess of Wales (Catherine) was evidently not over-happy about M asking to borrow her. She did give it to her but supposedly made a 'disgusted face' as M wiped some on her finger than on her lips.
I think I am in the PofW camp here.
Years ago a work colleague who I barely new, came to work late and asked me if I had any mascara she could borrow. It happened that I did have and I gave it her (a bit reluctantly) as I felt it wasn't hygienic to share.
I'd never ask anyone if I could use their make up where it touches your skin.