it's a little small but the colors and blending looks good from what I can see. The face doesn't look like it's supposed to be there because of the position. Try flipping the face horizontally so it looks like it's facing the same way as mona's head.
I think its a worthy effort, but by tilting my head on it's side my initial impressions are that the intensity and angle of the lighting may need some minor adjustments...The skin texture on her (in this case his ) face may also need a few tweaks.
Apart from that though I think you've done well and the colours in my view are spot on.
My head hurts ....................( I ) ...................Ω
I'm not an attorney, but based solely on what you've written, he most definitely does not own the rights just because he's used your work and not paid you. I mean, that's one of the stupidest arguments...