Did you ever speak about the situation with them? I mean, are you sure they know you are outside and they actually want you to hear that?
I'd like to give you my assessment on this:
I think they don't know that you are outside their door and maybe only hear some "noises" sometimes, which they think are random and more far away from all what's happening inside their room. So I'd assume that if you are going to actually go beyond this "line" of random noise, they will realize and probably the whole sitation will flip. If I were you, I'd use this as a constant source for recordings and not anything else.
I imagine if she would like the idea of someone listening and he doesn't (as much as she). What would happen if they split up someday? I'd assume that you won't be able to record him with a new girl once he knows about you listening.
These are just my thoughts how I would probably handle this different. However you are of course free to do whatever you like
Thanks for the recording though.
Best regards,
jinhui