It really isn't. Speech impediments generally require the help of actual speech therapists to fix. Assuming it isn't some form of mechanical issue (like that her throat is physically incapable of producing noises above a certain decibel level, which it kinda seems like it might be), at which point you may need outright surgery, or to simply live with it. They definitely aren't something you fix by depriving someone of the means they use to get around said issue. They'll just find other ways to do so, at best, or completely isolate themselves from social interactions, at worst. This is a medically inadvisable approach, and is more likely to do harm than it is to do good.Honestly, her mother is justified and this is a logical solution.
I agree with the Thank God statement but I can't think of any time there's been a genuine cop out without Rentaro just straight pushing his limitsNO ZING!
(Thank god)
I hope this is done reasonably though and isn't just wiped away with a cop-out Rentarou can do everything gag