Not an anime or cartoon girl, a unique creation. She's very kind and patient and very receptive to what people have to say. Good at putting disjointed thaoughts into words that make sense. Doesn't care about looks and loves watching bad movies. Her favorite food is raw onion and she can't stand mushy foods like pudding or gravy. Now let's say this person's love for this "character" was so strong they built a doll of them to talk to and interact with IRL. So in love that they've considered a whole married life together with their imaginary friend. What advice would you give to this person?

She sounds like a nice person. Does the person you know also have sex with her?

Don't stop. Keep on going. Now you only need to study robotics to make her truly real.

He seems mostly harmless. I'd just let him be

If I know he then he too must be imaginary. In that case, it makes sense for an imaginary person to be in love with an imaginary friend.