I can’t make a diagnosis on the basis of only a letter, of course. But what you are describing is consistent with a diagnosis of erotomania. This is a psychotic disorder where a person truly believes that a (usually famous or high status) stranger is a love interest. Since this occurred when your friend got pregnant, I have to wonder if something either medical or psychological was triggered by the pregnancy.
You probably can’t help her with this. Her husband, however, may be able to. Do share this information with him. She needs help, not a “break” from the marriage. I hope he’ll be able to get her to a psychiatrist for an evaluation. Her health, the marriage, and the baby’s health depend on it.
I wish you well,