How does the church view marriage of a woman to a man younger than her?
Marriage should not be based on the liking of someone. Your friend needs to assess what she likes about that person. Is she attracted to his looks, youth, or personality? One of the main criteria in choosing a life partner is the maturity level of that person in thought and behavior.
Usually girls mature faster than boys, depending on age range whether in their twenties or thirties.
This situation is not favored (but also not prohibited) by the church for the foreseeable problems that might occur after the marriage.
Encourage your friend to pray to God for enlightenment about this relationship.