Sorry, but let's be consequent - if there is only "your girlfriend" accepted, so why the only confirmed translation for "아들은 아빠에게 편지를 보내줄 거야" is "the dad", not "his dad" (or anyone else's)? I know it's more natural here but still, you can meet someone's (eg. mine, his) girlfriend as well.
This is a flaw in the program. The sentence about the son and father should have a more open set of possible translations. Flag that one when you can and hopefully it will be fixed soon.