How honest should I be?

I think that honesty is probably the best option. Anything else may lead to misunderstandings, possibly painful. Trust her to understand the reality of ME. Point out the research that said that ME is worse than MS for quality of life.

I was several years ago in a somewhat similar situation and know it didnt sound that great, but as longer we struggle with ME/CFS, as more friends we usually lost. There will be at last mostly the best friends left.
If we feel fatigued and tired, there will be always some friends that just think : "It is only in his head".
They just dont tell it in this way, because they dont want to disgrace or
to stump your head, so they sometimes prefer sentences like:
"I also feel tired if I work too much, why do you not sleep a little bit more and try to relaxe - you will see that you will be back, soon"

We can explain what ME/CFS is and our best friends will mostly unterstand us, but there will always be few who never get the point, exspecially if they start to "Google" --> they will quickly find sources "here & there" that claim it is a psychiatric illness. So sometimes few ppl. just feel sorry for us, if we dont seek for psychiatric help, that we dont see "the psychosomatic cause" - I dont found it any helpfull if ppl. think about ME/CFS in this way.

So I would say it depends on the friendship how honest we can be about our illness. On the other side, I want to note that I had friends that I always thought that they were not my best friends, but after they heared that I was getting ill, they became one off my best friends. I was never in need to explain them anything about my illness, they were just good friends who helped me.
Sometimes friendships can be mysteriously like ME/CFS.

"Ive been struggling the last few weeks, but I do my best. This weekend will try to get out for a bit. Thank you for asking, how have you been?"

I normally will lightly touch how im doing. End the sentence/subject on a posiitive note, and bring them back to the conversation with a bit more focus on them.

I agree with honesty being important however if you define yourself as "the sick lady" then that will become your identity. People generally find the sick to be downers (especially when we have mental illness) and have a natural aversion to them so be judicious about how much you complain.

Umm....friends and some family members, how much to tell them. I'm 73 now, and have had a lot of experience with both over the years.

I never told my family just how bad things were for me, only b/c I didn't want my mother to find out about my primary conditon and for her to know that she was the actual cause of it. Not knowingly, of course, but b/c of lack of vitamins and food in England during WWlI. (She was a war bride from London and moved to a small place in Pa.) So, none of them, except a paralyzed brother knew. Why upset someone with something they didn't know about and couldn't avoid? That's cruelty in my book.

Others moved away, could never understand, but never made me feel like a liar, etc., but then there are those who could be trusted with anything. They knew before my illnesses and we each wanted to stay in each other's lives. I recommended books for them to read, the docu-drama, UNREST and a wonderful one done by a young man who also has it....he interviewed fellow sufferers and how they would get out and then keep falling back into the black hole that ruined their lives. There are also those who won't give in to anything, and I suspect that I'm one of them. This was a great disappointment to me, but sooner or later everyone has something and must take the heat and mold themselves into stee .

Anyway, I've heard of friends that you think would be your staunchest allies avoiding you, and others on the fringe who have made a tremendous effort to stay in touch. But we have to do our part, too. and don't dwell on our illness. You need a break from it. Listen to them talk about what's been happening in their world. Tell them you want/need laughter in your life and would welcome anything they have to say. Find something to love...a pet is what some people do. I have great empathy for everyone I have contact with. As we age we realize that everyone does carry their own private sorrows and pain. Let things slide, as you watch a game or just talk with your friend, the person you want to keep in your life. And don't give up hope of having a mate in your life. To keep a friend; be a friend. It's so worth it for both parties. Yours Lenora.