Use any and all natural lighting around. Lampposts, large neon signs, even moonlight on a clear night if its full enough.
Use the speedlight, set it up on a tripod, set the camera to 1/20 and the flash to 1/128 or 1/64
The idea is that any "clarity" of image and facial features is for that split second (128th/64th) lit up, and the camera's shutter speed of 1/20th is there to soak up ambient light.
You'll need a diffuser or you'll have some harsh-ass light on facial features. If she wants to absolutely ditch the flashes too, you need a tripod.
There is no fucking way in hell you will get a single steady shot where not one of you is moving, either the camera or the subject themselves.
Also pump ISO to 1600+, embrace the noise, cuz it will fuck you a new one.