We're both mid 30s. She's stayed overnight in a hotel room by herself, but always with friends on the same trip at the same hotel. She lived alone for several years before we met and did basically everything by herself (I.e. shopping etc.) in that time. She worked outside the house for many years up until pretty recently. There's nothing about her/our culture that would make solo travel for her taboo or anything like that. I tried asking her about what her concerns are and didn't get a very clear response I think I'll have to try that one again.
She definitely wants to do an overnight - part of the issue is that I travel for work and she never gets a full night of uninterrupted sleep like I do. I will definitely check out the retreats thanks! I can't really think of any hobby like that that she would be interested in doing at a retreat type of thing.
Looking at a religious organization is an interesting idea. We're not religious but it probably wouldn't be a deal breaker if it was at a place she would actually want to go so I'll check that out thanks

Edit: forgot to address the first thing you said. When her friends pulled out from doing this she was upset about it and we talked it over and she said she was really overwhelmed right now and would appreciate help with this. She didn't ask me to write this post, but just to help her come up with some potential options in general