Great story! Thanks for sharing it.
Don't know about Mary. There are over 5000 people whose name could fit with some variation of Mary Rice in WikiTree.
978 of them were born between 1822 and 1846, which would make her age 18-42 at the time the letter was written.
259 of them had the exact surname of Rice.
158 of them had the LNAB of Rice, (since school teachers were usually not married.)
None of them were born in Massachusetts. (assuming that is her birth place.)
So, unless we can learn more about the place and exact year she was born, it (sadly) seems that she is not in WikiTree.