Because I read the English sefer The Fifth Commandment and it definitively says that naming after one's parent is part of the mitzvah of kibbud av vem.
I would tell op that if it were any one other than a parent to hold her ground, but since it is a parent, that name takes precedent.
I do believe she will learn to tolerate it.
It's not a chiyuv in naming ones child. There is no such halacha in the shulchan aruch.
Yes, it can be a way to honor a parent. But there is no halacha: "when naming a your son at his bris you must use your father's name if he is deceased". It's just not a halacha.
(In my community it's not a such done thing to name after grandparents...some do, some don't...)
