Sound carpet

Meet her in the middle with a rug that's halfway as thick as your shag carpet and twice as thick as her thin one. If all it took was going down in thickness then coming back up some should do the trick.

All the best,
Nonoise

You can often compensate for missing room treatments in other locations.

If that was the extent of your room treatment, get wall mounted and ceiling mounted panels.  GIK makes some Art Panels that let you have anything you want printed on them.

@triskadecaphobic Although I haven't seen any deep testing/analysis to prove it, I've read that natural fiber rugs are absorptive, while most man-made fiber rugs are reflective and don't help much. Perhaps a natural fibre rug in whatever thickness works for you both will do the trick. 

If you're more interested in "using a pair" than "growing a pair", ask her to pick one out that meets your need. ;-) Cheers,

Spencer

+1 sbank, a natural wool rug will absorb better. I've been told that by a recording engineer. He recommended hanging a wool rug behind my speakers (it vented out the back) to keep the sound from transmitting down to my neighbor below me.

It did the trick.

All the best,
Nonoise