Apart from the other suggestions (ie, not Hi Tec cheap boots, get a size larger than usual to allow for swelling and proper anti blister socks), if she's got flat feet, she really needs insoles, as part of the issue could be movement from her foot collapsing sideways and putting pressure on the wrong rough bits of the cheap footwear.
To get seen quickly, a private Podiatrist appointment will make it possible for her to have appropriate insoles, they'll encourage her to accept that discomfort is important to take notice of before it becomes pain or an injury (sometimes you need an 'expert' to tell people things, rather than just Mum) and then, after a period of gradually increasing the time of wearing the orthotics/insoles to gradually get used to them, her weight will be properly distributed over her feet and the mechanics of walking will be improved to the extent that blisters or other injuries are far less likely.
To give an example, whilst my actual shoe size is under a 6, I have a size 8 in running shoes and a particularly wide toebox, as that allows space for my foot to spread under load/swell after a long time on them and my orthotics to fit in. I've not had any blisters, redness or soreness since I sized up, but beforehand, I would regularly cover sections of my feet with dressings knowing that they would be rubbed sore fairly quickly and red raw if I got wet feet and had to keep on going.
If you can't stretch to Podiatry, I'd suggest that you take DD to a specialist running shop where they can measure her feet, make recommendations for trail or fell running shoes and possibly gait analysis where they can show her/you what needs to be corrected or allowed for.
When she does need to do the main walk, I'd also suggest that she prepares for it by putting white adhesive dressings over her foot beforehand - the giant white plaster style are really good for providing a protective surface and stay put until they need to come off again - and include the sides of her little toes and possibly the ball of her foot as well. It may seem like overkill, but it's so important to pay attention when there's something wrong immediately, rather than wait until there are giant sores or socks filled with blood.