How can I help person A? She can't physically leave person B because she is basically his carer. He doesn't treat her well and she can't live her life because he is her number one priority. She can't come and go as she pleases let along leave him. For the last twenty years that she's been with him she's helped him with everything and I don't know how long she can put up with it. He hasn't been out the house properly for four years and either person A or person C has to be in the house with him every day. They had a gambling problem but they have done this thing where they can ban themself. Apart from that, it was the only they went too and something they could enjoy. But they would always spend all their money and it couldn't go on anymore. Person A doesn't know what she can do, it's like she's trapped!
From this it sounds like person A is in a sticky situation. It's understandable for person A to feel trapped and partially person C to feel a little trapped them self too. I feel like anyone who is a carer for someone has to do what they have to do, it's part of their life and becomes mundane especially after twenty years. However this shouldn't mean that person A or person C should not have fun themselves either.
My advice on this situation is to take time to do the things that you like. Possibly a daily hobby could be something that could be done in the home whilst person B is well occupied. This could be something like painting, reading or even possibly playing computer games. This way it would give person A and person C something that they can do to take them away from the stresses whilst not having to feel guilty about it. Then possibly once a week person A could go out with friends whilst person C looks after person B and vice versa.
If this is too difficult as person B is in a critical condition. I would recommend looking into some charity's in your local area where carers can go to meet other carers and have a slight break whilst person B is being looked after. This would be a great place for person A and C to go so that they could meet new people in a similar situation as them. This would mean you could get advice from them as well as providing some sort of laid back counselling.