I'm back from a morning networking for your mum and here's what I found out.
A friend, v.v.smart who adores her clothes (and she's 80 and size 42DD) showed me her latest 3 bras. satin/lace, straps wide enough to give lift and support, she'd got white, lilac and mocha- 18.00 each, M&S. They would v.easily adapt to be pocketed. Btw, for DIY pocketing (the way to go, I reckon, not just cos the NHL will only do ONE for you and it's an expensive service they offer but also who on earth wants to be restricted to just one bra??) -anyway, the material needs to be something like soft jersey cotton. Regular cotton too harsh, I think.
Friend's sil had a mastectomy a few years ago and she buys her bras at Asda and does her own pockets too.
Hope this helps. Was really impressed by the pretty M&S ones. I know that some stores also sell bras for mast.customers but back in the days when I was looking there didn't seem to be a very inspiring choice.
Has the H introduced her to Amoena yet? Skinny perfectly toned young models and I'd bet that underneath their bra forms they've got the full works......spit.
Again, all the v.v. Best to your mum from me. X