Should religion be a "deal breaker" in a relationship?

I have a friend who has been dating this guy for 2 years now. His family is very into their religion and so is he. However, he doesn't always follow what his family does. His mother would like my friend to convert or be more interested in the religion. The mother used to approve of the relationship when my friend was interested in learning about their religion, but about a year ago my friend said she does not want to study it anymore. Since then my friend has not been to her boyfriends house or even seen her boyfriends family. I keep telling her this relationship can't go on. The boyfriend says he doesn't care what his mother says, because he wants to be with my friend forever.
I want others opinions!