room treatment gurus, please

Glass walls are the worst. A good room is a mix of absorption and diffusion.

DSP won’t fix 1st reflection points whereas your panels would have.

I was using dsp for a couple years trying to get the sound right with my speakers that were just too big for my current room. Buying speakers to fit my room, adjusting the speakers using the techniques from Jim Smith and Cardas, I stopped using dsp and allow me to put up for sale some of my absorption panels.

Drywall with Cavities are the most difficult to work with. 

A Filled Cavity is the better of the Two Evils, the room coupling of Speakers is quite difficult in such an environment. 

It was described to me, by an individual who is quite skilled at producing Speakers, it is due to Drywall Systems with Cavities, that Typical Speaker Designs produced in the US are heavilyy augmented in the Bass Frequency.  The same individual stated that when such designs arrive to be a sale item in the UK, when used in a Typical UK Home the Bass is noticeably overbearing. 

A corner set up can be effective in a square room. You could also consult an acoustician. Jeff at hdacoustics is very good and I highly recommend him. Good luck!

You didn’t note the speaker and listening chair placement. I am stuck with a difficult room but have found inches of speaker movement can make a significant difference.  Same with moving the listening chair. It might be interesting to create a near-field space in the middle of the room temporarily (like at a mixing board) just to hear what’s possible. Also, the corner configuration mentioned above solves a lot of problems. 
 

Has your friend tried damping the rear wall to hold down standing waves?  Or any other experiments with the room?  Rugs on the floor?  A little bit of absorption can make big changes. Of course, if you can have an acoustician treat the room, all the better. I have found the experimentation interesting and kind of fun. It’s helpful to have friends with good ears who can provide feedback. 

You can still enjoy the system by using the right volume, not to overwhelm the room.Maybe you can use room divider that folds .