It's a nice thing you are doing OP. Although I'm not your mum's age, I am certainly an older woman who has recently lost a lot of weight, and gone from a 22 to a 14. What I will say is that my attitude to clothes and style has changed enormously via that process. I'm still fairly relaxed and comfort driven in how I dress, although that's in part due to limitations on what I can wear on one of my feet, but I have an interest in clothes now that surprises even me (and shocks the rest of my family!). I'm still losing weight so my wardrobe is still transitional, but essential for me was firstly good foundations. So being properly refitted for bras and some pretty but functional underwear to deal with the crinkly skin which is a pretty much inevitable consequence of that amount of weight loss, especially when you are older was essential (but not necessarily something your mum might want to over-share!)
Secondly, I had a good think about how I wanted to FEEL day to day, and what colours I liked. Then I phoned for loads of paper catalogues - most companies still seem to have them, which surprised me: Boden, Pure Collection, Hush, Landsend, Poetry London, Wrap etc. I cut out looks I really liked and reassembled them and then went shopping with these looks in mind. The old-school version of Pinterest, I guess. 
. I only ever shopped online before, but now I do enjoy holding stuff up against me in shops, and I find a big John Lewis does serve the purpose well.
My transitional wardrobe is:
3 bras from bravissimo - unless your mum has very big boobs, much cheaper options on the high street, but a bra intervention from MN would help get the size right.
7 pairs of pants from M&S ( the very light shape wear ones, mainly)
2 pairs legging from Sainsbury's (the £10 ones)
Thick black tights from Primark (love the fleece lined ones)
Two pairs trouser leggings (like slim fitting trousers) from uniqlo (at a 14, I'm wearing an L from there)
Down coat from Uniqlo (again in an L)
Long sleeve tops in a mix of colours (uniqlo and next essentials)
Two slightly oversized cashmere jumpers (Boden)
Two Long boyfriend style cardigans (M&S) in a medium
Tunic dress from Poetry
Two tunic dress from Hush in a medium (other good options, especially if your mum likes patterns are Joules, White Stuff, FatFace)
Jeans from JohnRocha at Debenhams
Vest tops from M&S for layering.
New pyjamas!
Add in some lovely trainers and some new costume jewellery, and I was good to go.
Hope that gives you some idea of the kind of things I bought, even though your mum will obviously have her own preferences. I have added some 'treat' clothes as well, but these were the basics, and clearly there are loads of both cheaper and more expensive options out there. This wardrobe takes me from a casual work environment, to socialising to dog walking. I like a charity shop rummage, and am using some stuff from there to experiment with my style, but initially, I just needed some basic clothes that fit me well.
Hope that helps. And a huge well done to your mum - I've lost 50lbs so far, and it has transformed me. Hope it does the same for her. :)