Post a new message on the "New England Historic Genealogical Society" board - that is a subscription based online info of BMD for the years 1850-1910.
Ask someone to do a lookup for you for:

Ambler, Mary Louisa(Gray) Medford, MA Year=1900,Volume=505,Page=603

That record will give you the information on Mary's death.

I found that information from online index at Massachusetts Archives: http://www.sec.state.ma.us/arc/arcsrch/SearchWelcome.html

You can now search for births for Mary Ambler by putting in last name = Ambler, start year=1877(year of marriage), end year=1900(Mary's death). You can also put in a town but it looks like they moved a little. You would then post another message for someone to lookup the birth record.

Good luck