Her boss is wrong. A calendar month is from a certain date in one
month to the same date (actually the day before) the following month.
So you daughter is correct, her employment should terminate on the
16th April (actuall the 15th as the first day is classed as the first
daye of the notice).
In the 3rd edition of Halsbury's Laws of England, volume 37, the
authors wrote:
"When the period prescribed is a calendar month running from any
arbitrary date the period expires with the day in the succeeding month
immediately preceding the day corresponding to the date upon which the
period starts: save that, if the period starts at the end of a
calendar month which contains more days than the next succeeding
month, the period expires at the end of the latter month."