I'm concerned about my neighbour's cat. The cat seems to be out all day and night (even during last winter when the temps dropped to -10), and from what I can see, is never fed

I don't want to interfere, but I'm really not convinced that this cat is being properly cared for.

Should I phone the RSPCA?

Only you can make that decision as we cannot see the true picture to help you decide.

Are these people approachable enough to talk with and drop in a few subtle comments perhaps?

Maybe you could even put a little shelter in your garden with some food/water if needs be.

Of course if you really think this cat is suffering and is neglect maybe consider ringing the Cats protection for advice and even the RSPCA to see what they say on the matter.

Can you stroke the cat? Does it feel thin? Does it eat ravenously if cat food is available? If it doesn't it is unlikely to be hungry although if it does it may just be a greedy cat or if you offer fish or chicken many well fed cats would still grab it up.
Does it have any shelter outside? Some cats are feral and prefer to live out.
As the poster above said only you can judge if it is suffering or being neglected, although be warned, the RSPCA are not always as helpful as you might hope.

Does the neighbour have a cat flap which allows the cat to come and go as it pleases or is it locked out all day and all night? If there is a catflap it may be the cats own choice. Try to find out these things before you do anything but if it is being locked out and simply can't get access to food and shelter when it needs to, I'd be tempted to go with what has been said already and maybe make a little shelter with food and water for the immediate problem then think about what should be done long term. If the cat is being wilflully neclected call Cats protection for advice because I also agree the RSPCA aren't always as helpful as you'd expect them to be.

The cat is quite thin. The neighbour doesn't have a cat flap, and the cat is always crying to get in.

I feed her from time to time.