If they're willing to go via LAX then you can use UA miles on Copa Airlines, which has a nice premium service to Lima via Panama City. Not lay-flat like United but more direct at better travel times.

You can get a free stop over in Lima with a round trip to Cusco on TACA, also in First Class, all with UA miles. We did this last year (from ORD) and it worked well.

From there it was pretty easy to get to/from MP and visit all around the Sacred Valley. We paid extra to book everything ahead through a US tour agency for the peace of mind. In retrospect it appeared pretty trivial for any confident, adventurous person to wing it when there because it is all a pretty well-oiled tourist machine.

I'd recommend spending a couple days in Agua Caliente to see MP more than once. I had pretty high expectations and MP still exceeded them, it was that amazing.