A good handyman should be able to install it correctly. The only thing different from a horizontal AC is that you need to fabricate a shelf to rest the AC on. I don't know about the one she bought but when I bought mine (Kenmore) it came with the shelf and associated hardware.

You could ask a rental property owner or superintendant for references.

I am not sure how much a window would cost to install in the DFW area, but $500 seems a bit much.

Is there a Best Buy store nearby? If so ask how much they charge for installation. If they have a model that will work for your window then return the one you bought from Home Depot. You could ask Home Depot to match Best Buy's installation price, but I am not sure that will work. Home Depot uses third party companies for installation and the prices are usually fixed.

UPDATE: Finally was able to get the Frigidaire a/c wanted without dents but had problems finding someone to install. Yes, $500 was too much, however I think that price quoted because they did not want the job. Others never returned calls. Finally a husband/wife team handyman accepted the job and charged $142. They did a great job. The electrician to run a new wire/outlet just for the a/c qoted a price, then did not return calls. Found another electrician to install the wire and he did a great job. No one wants to do small jobs since the building industry is going full swing these days.

Great news! It seems every small business is over loaded with work. A little patience eventually pays off.