I work in HR and think she's been dealt a raw deal and may well have a case. Things like this really annoy me, so apologies for the length of the post!!
So taking each thing in turn:
Contract of employment-as a pp advised, whether she has a physical contract of employment is irrelevant. In employment law the courts would look at whether a contract can be 'implied' ie whether it looks to exist or not. I don't know all the details, but from what has been said I assume she has been 'treated' as an employee for 10 years ie her employer pays her tax/ni (she doesn't do her own tax return etc) and she has had regular work from them -ie doesn't need to ask whether to come back the next week like an agency employee may need to do.
If so (and I've massively simplified it above) she is an employee of theirs whether they would like her to be or not.
As such, and because she has more than 2 years service she has many employment rights. She basically defaults to the basic statutory terms and conditions under employment law I.e. Can claim statutory redundancy pay, sick pay, a notice period etc She is also protected under law against dismissal etc.
Incidentally, they should have provided her with written terms and conditions (essentially a contract) within the first 12 weeks of her employment and without this she can make a claim to the courts for their failure to provide it. She can always keep this info in her back pocket if they turn nastier.
Changing her Hours: They can change her hours, but would need to consult with her first, which means that they need to give her appropriate notice and reasons for the change. These reasons can't be related to her mat leave or her SEN child they need to be for a business related reason. (I.e. Are they changing their opening hours, efficiency reasons etc) I agree with pp, she needs to request these reasons in writing and challenge why she hasn't been given appropriate notice to change her hours.
Walking difficulties/early mat leave: in some circumstances an employer can force someone to go on early maternity leave, but only where they are off sick for a maternity related reason a few weeks before they are due. I imagine that if she was struggling to perform the role then they may need to have a conversation with her.
But the first thing they should do is to understand the condition and put what are referred to as 'reasonable adjustments' in place. This means that they are obliged to adjust the role (temporarily) eg give her a chair, reduce her stacking shelves duties etc, they've clearly not considered whether they can do this, or implemented anything ....
So, they are massively on dodgy ground by not 1- writing to the GP/consultant themselves to understand her condition and have just assumed they know she can't work and 2- forcing her to start on mat leave early without doing this. Appreciate they are a tiny employer, but they need to be able to show to a court that this has been considered.
Next steps:
Agree with you that she should seek some advice as otherwise it sounds like a case of the blind leading the blind as her employer sounds utterly clueless and don't know what to do. If CAB doesn't work, then she could try-calling a law firm directly, most of them will give their first hour of advice for free (you just ring up and ask and they'll put you through to someone) or, she could claim on her house insurance, which many people do. The other alternative is joining a union, which I imagine she's not in, I'm not sure which Union is associated with shop workers, but if she googles it she will find it. Some would be happy to take a non-member (if they start paying their subscription) some wouldn't, but it might be worth a try as a union would be a cheaper alternative to a solicitor.
Grievance-I would advise that her next steps is to write a grievance letter to her employer. Basically set out in writing all her concerns in a complaint letter and ask them to respond to each point. They (should) then hold a meeting to discuss this with her, and then she has a right of appeal. She may need some support to draft this (hence the union rep/solicitor etc) and a union rep would be allowed into the meeting (and any appeal) to represent her.
The point of a grievance letter is to formally document her concerns, some people worry about raising concerns, but in her case I don't think she has much choice.... And in my experience, employers are sometimes a bit rattled that they have been found out, but respect people who raise things.
As I said at the top, I hate things like this (hence the massive post!) and urge people to raise things when they are being treated unfairly! So best of luck and hope she gets it sorted.