I trust tptacek that WhatsApp is end-to-end encrypted. But it definitely leaks metadata. Nobody is denying that.
Facebook now knows gf is communicating with a lawyer.
Your threat model may vary, but my assumption is Facebook will sell that info to anyone who pays. (Not directly, but definitely using some sneaky ad targetting: all girls, 27-35, this zip code, this education, etc, and who has contacted to a lawyer recently.)
If this isn't possible it is just because they are so busy doing all kinds of worse things that they haven't gotten around to it yet ;-)
I'm only halfway joking: this is the company that fools people who try to be secure into giving up their mobile phone number, then shortly after starts using it for targetting.