I bet the boy has NOT forgotten about her, perhaps you can take a picture of her in her new home, and mail it to him with a simple little note, so he will know she is happy. He is young, so is probably not allowed to use the phone by himself, and the parents may not want him to "bother" you.

If he knows she is happy, it will let him feel a teeny bit better about the whole thing, so please do!