Remind your friend that prior to Vatican II, Latin-English missals with side-by-side texts were used at Mass, so the faithful could follow along and not miss a word. Usually the priest read the Gospel first in Latin, then in the vernacular. Sermons were given in the vernacular. If the Church didn’t want anyone to understand what was being said at Mass, then everything, including the homily, would be have been spoken in Latin, and the people would not have had missals to follow during Mass.
Latin was used in the Mass for centuries because it is the official language of the Church. Cardinal Arinze says that Latin "suits a Church that is universal. It has a stability modern languages don’t have . . . "
In Mediator Dei, Pope Pius XII calls the use of Latin in the liturgy "an effective antidote for any corruption of doctrinal truth."
The use of the Latin language, customary in a considerable portion of the Church, is a manifest and beautiful sign of unity, as well as an effective antidote for any corruption of doctrinal truth. In spite of this, the use of the mother tongue in connection with several of the rites may be of much advantage to the people. But the Apostolic See alone is empowered to grant this permission. It is forbidden, therefore, to take any action whatever of this nature without having requested and obtained such consent, since the sacred liturgy, as we have said, is entirely subject to the discretion and approval of the Holy See. (MD 60)