Hi, sorry to hear about your friend.
If I were in your shoes I'd like to take a look at at the scene ahead of time - decide where you might like to place them, where you might shoot from, and what the background is. I'd be hoping to have them in some window light with a darker, somewhat unobtrusive background. Perhaps the BG shows a wall of older family photos, etc., and perhaps you want to move some furniture, etc. The time of day will affect the window light, so this is part of the reconnaissance trip. You can also judge whether you want some supplementary light or a reflector. I don't know how comfortable you are with lighting things yourself, but if no experience with it, I'd suggest to either do without, or keep it simple with a reflector, or perhaps just move one of their lamps closer (with B&W there are no color imbalance issues).
If you have them facing somewhat into the light, and you are shooting from off to one side of it, this will tend to "narrow" the faces, which is, I think, one of your big questions.
I always try to work from a camera stand for more serious portrait work. You can set up the camera, focused and framed roughly how you want it, then you're free to talk and interact with them, just clicking the shutter release at what you think are the right moments. To me, this is the essence of a portrait - the lighting and setup are sort of the window dressing for this.
I'd want to see the two of them sharing moments, perhaps reminiscing about days gone by, perhaps looking through a photo album, etc. This sort of thing, your photos of some shared moments in their home, will, I think, give her some comfort should he have to go into a nursing home (or other bad things). In fact, just knowing that she has these photos to reminisce over will probably give her more comfort than you might imagine. It's a really good thing that you're doing.
Ps, I'm guessing that he'll be in a wheelchair; if so a lightweight blanket on his lap can be draped over shiny parts, etc.; these can be really distracting. Best of luck with your efforts; I'm sure they'll be pleased irrespective of how things come out. I almost wish I could stop by and help you out; I've also seen family members going through similar.
