HI there, I have a mastectomy and I use online companies (Nicola Jane, Contura Belle for example)
I don't especially like M&S range of bras - because the straps are too narrow, - though they've got a couple or plain ones which are OK)
John Lewis also do some, (Rose Royce I think - also online) and do mastectomy swimwear.
Your hospital will put pockets into a swimsuit for you, as will John Lewis (if you buy an ordinary swimsuit from them) but I find mastectomy swimsuits better because they come higher up your chest and underarm, so the scar doesn't show.
Nicola Jane and others often have sales in swimwear .
I've got a Been-a-Boob I use for swimming - it goes into the right shape and doesn't get waterlogged. Another tip is once she gets her proper prosthesis from the hospital, she can sew the 'softie' inside the bra part of a swimsuit - so it won't 'escape' 
I found a tiny safety pin hed the softie on place in my bra.
It is worth looking online to see if you have an independent specialist store in Birmingham - they will have a good choice and will help with fitting etc. Big cities sometimes do have them.
My tips for bras (obviously just iME) are wider straps are more comfortable, and I like bras which have a lace insert across the chest part - because it hides my scar, and stops everything showing if I lean forward.
I also found little vest top with a lacy bit at the top handy for popping under lower cut clothing (cheaper alternative to revamping your whole wardrobe)
HTH If I think of anything else, I let you know
If you or your mum have any question or anything - or just feel like a chat, - there is the Tamoxifen thread - all ladies who have had BC, lots of support there and you'd be very welcome.
Best wishes to your mum - I hope she finds something to suit