I'd totally do what you're doing. Your mother needs you very much and I'm sure she knows the effort you're making her to keep her happy and comfortable in every way possible. It sounds like she is a great lady.
People like her friend will be in all areas of our lives, so it's up to you to be the adult and your mother sure appreciates you behaving like one. Kudos! You're being a good son =)
First things first- I'm sorry to hear about your mother's illness, it is awesome that you have the opportunity to be with her to lend support. Second things second- to hell with your ma's friend, I too have family which believes that so long as they have a faith in the supernatural they have it all covered, if she had any sense, she would be proud of her kids for real accomplishments, if they have any to speak of, obviously. It is most likely that your mother simply appreciates what you give of yourself but sometimes loved ones who are believers do in fact worry themselves about non believing family members- that is not your responsibility. I would think that being honest and treating your mother like an adult is more beneficial to all involved than to lie and compromise yourself. Just because your parents are believers doesn't mean they won't know if your lying to them, they are still your parents.
First off, thanks for sharing.
Secondly, I just wanted to express my condolences for what you and your family are going through.
I guess the only real advice I can give is this:
Your mother's illness is and your current living arrangements are a very temporary thing. The memories you will have of her are not. How you handle yourself and your situation now will be carried through your entire life with a mixture of many, many emotions. This might be a defining moment in your life. Proceed with the utmost caution.
Right now you are walking a balance between self honesty and true compassion.
I do not envy your position.
Just know that you have friends here that have been through what you are experiencing.
I can't provide you with the answers, Cowboy...but I can provide you with support.
There is an entire community here that has your back.
If you ever want to talk, please email me. If you ever need a human connection I'm happy to send you my phone number or video chat.
No one knows your family like you do. No one can make the daily decisions but you.
Just know that for good for for bad, there are people here that care.