Legal/tax/moral issues aside, unless your "guy" has a license and insurance, and is in the business of being an independent contractor, you would sure be asking him to hang his butt way out there doing this. Of course, if he's judgement proof, he may not care. And, if he's smart, he would agree to bill you at the shop rate, then pay you his hourly rate. Only fair.
As a long time small business owner, unless there's something I don't know, asking an employee to do that, would feel rude to me. Not dumping on you, just bringing up something you haven't considered yet.
And, yeah, masonry flue, don't do it. If you really want one, I have two, and I'd be happy to send both to you. Class A just works so much better. Use your skills to go as wild as you want around the stove, but do yourself a favor and incorporate a Class A tube. I don't think a day goes by that I don't think about my masonry without also thinking of a jackhammer.