What strings were you using on the guitar before? Why not just go back to that brand? There shouldn't be any need to stuff a towel into the guitar.
You should also think about what it is about your wife's right hand attack that's making the guitar sound brash and harsh. If she's using a pick, experiment with different pick materials. Dunlop makes nylon picks that have a fairly mellow sound:
https://www.jimdunlop.com/category/p...picks/nylon.do
They also make picks out of felt, which are even mellower yet:
https://www.jimdunlop.com/search.do?query=felt+picks
https://www.sweetwater.com/store/det...lt-picks-12-pk
The felt picks are mainly used by ukulele players, it seems, but would work just as well on nylon string guitars. They eliminate the clacking sound that plastic picks make on nylon strings.
In short, before you go changing too much about your guitar, check whether it's really the guitar that's the problem rather than the way it's being played.
If she doesn't use a pick, then we're back to where we were. But if she does play with a pick, using felt picks might help a lot.
Hope this helps.
Wade Hampton Miller