hmmm,,,, while this obviously isn't tracing, personally I'd still feel really disappointed if I received a commission like this.
The pose and the angle aren't just similar, they're virtually identical. I'm not saying you should never reference other peoples art, in fact, you totally should. but not when somebody else is paying for your creativity. use the same pose, by all means, as the others have pointed out nobody can own a pose. but I'd use a completely different angle at the very least. If you can overlay your art onto another person's work and the lines themselves are still like 85% similar (even without tracing), I don't think you're being entirely honest with the person paying you.
Ethics aside, copying other people's art shows in the final product, there's a certain flimsiness that happens when things are copied without being understood in 3D space. Not just in this example, but in general.... a discerning eye would be able to pick the pieces in my own gallery that are directly copied from single references, too. (this is really obviously copied from this one photo, for instance)
Please understand I'm not saying any of this to try and be mean! I earnestly want fellow artists to push themselves to improve, and while studying and copying other people's art is an important part of that process, I think paid work is the time to flex your ability to combine and re-interpret your influences, not just copy from one. Try combining multiple references instead of just one, or imagine a reference from a different point of view.
Anyway yea that's my 2c .....As for this piece, the choice is still yours--it's not like you've committed any crimes here. Your client might not mind, or they might be fussy. Maybe you could show them this overlay, and ask them their opinion?