Hey Neill,
Save your relationship, we see loads of guys trying to teach their girlfriends and it almost always ends in tears. Same goes for dads teaching daughters.
A girlfriend will really "try one on" with her boyfriend when things aren't going as easily as she would have liked, whereas she will not do the same with an instructor and will be more open to critical feedback.
When the going gets tough she'll get criticism from her BF and critical feedback from an instructor, the difference is in the relationship dynamic. Most girls won't play the "I just can't do it" card on a stranger (instructor).
I know of people who have taught their partners successfully and I guess that depends upon the dynamics of your relationship.
Let her learn the basics on the specially adapted gear in a quality school and after a couple of lessons, when she has gotten up on the board for the first time, then you take over. Learning relaunch, self rescue, water start and upwind body dragging is better done by a school whilst you are out ripping it up.
Just my 2 bobs worth, have fun either way and don't push her into it, let her take it up at her own readiness.
Good winds,