It seems to me that Aleksandra believed in many quacks and quick remedies. Who knows what horrors she could have put herself through with those treatments?
They mustn't have been too bad, though, because Maria would not have been a witness, then.
But, Maria seems to be familiar with the treatment in her letter because she writes it as "the" treatment, which suggests familiarity, something typical.
If not, something that was talked openly about earlier, or even argued about.
Of course, if there had been an argument about that, suppose with Alix and Nicky, Maria would not have been an attending party.
In conclusion, this treatment seems to be a routine, that the family knew about, nothing dangerous or new.
Sofia