What is the ruling on issuing a fatwa regarding telling a friend—in support of her commitment to prayer—that her deceased friend had enjoined her in a dream to commit to prayer, even though this claim is untrue?

The fundamental principle for a Muslim is to avoid lying, unless it becomes an unavoidable means to a legitimate objective that does not cause harm to others. However, it is preferable to avoid it by using tawriya (allusory speech). For example, you could tell your friend about her friend, as if it were you in a dream, and remind her of death and preparing for it, and encourage her towards righteousness and prayer.