There are a string of posts about this entire situation going back some months, in some of which it is your friend, and others it is you. It would be helpful to know which it is because, with the best will in the world, whether the information we are seeing is being filtered through a third party matters. If you are hearing a version of events from a friend, you are likely hearing only their side of it. If it is happening to you directly, then it is still only one persons side of events, but it is also at least the direct version of one person.
There seems to be multiple issues here but the employer does appear to have made attempts to support you / your friend. They have moved her/you to another store because you said that you could not cope with how busy the former store was. She/you have been reluctant to properly engage with occupational health when the employer asked for an assessment. But then, in a smaller and less busy store (with presumably fewer staff), having got what she/you asked for the issue is now that they won't give more flexibility on shifts and she/ you don't like working later shifts and with no other staff on. And now she/ you also want to quit and want to know if doing that will affect your / her UC.
Having all this and more information seeded across multiple threads actually makes it harder to give you good advice because nobody is seeing the bigger picture. But if the employer has done quite a lot to support you, and if that isn't helping, for whatever reason, there will come a point at which you / she must decide to either accept this situation or to move on - whether that means finding another job or not. As individual circumstances, some of these events may seem that the employer is being very unreasonable. But taken together they paint a different picture, and I do wonder to what extent her / your underlying problems are colouring your /her view of what is happening. Perhaps the manager is being a bit short on this occasion. But equally, perhaps she is struggling with an employee who, reasonably or not, always seems to want something different / more, and this was the last straw.
Managers are only human. They can only do so much. Perhaps it might be better now for you / your friend to consider whether a fresh start somewhere else will help. Whatever the circumstances are, and regardless of how you/ she has got to this situation, it doesn't seem like there is a way to easily resolve it. When that happens, a fresh start can be the best for everyone. That doesn't minimise the anxiety you / your friend is feeling. But sometimes anxiety doesn't tell us the truth, and our reactions to the perception we have are uncontrolled, making it all a vicious spiral.