Has she rung buyers directly at some of the big retailers? IME, buyers are pretty busy (or like to make out that they are 
) so any letter or email to them is likely to end up at the bottom of a pile. Likewise, writing to HR will probably just result in a standard 'Thank you for your interest, we will keep your letter on file...' response.
I'd get hold of the main switchboard number for some large retailers, phone and ask to be put through to the X Buyer (e.g. Children's Books Buyer, Wine and Spirits Buyer, whatever) and if she gets hold of them, do a 2 minute pitch. Ask them if she can follow this up by emailing her details to them. She'd have to be very clear on what she could contribute so they don't think they'd just be babysitting her for a few weeks.
Don't ever phone on a Monday morning, this is when buying teams run their sales data from the previous week and it's always a bit fraught.
Actually, quite good times to get buyers at their desks (again, IME) is really early in the morning (pre-9am) or late in the evening (post 6pm).
She might have left it a bit late, TBH, but there are opportunities. I know George
(clothing part of Asda) have interns from time to time, I'm sure others would too.
Hope you get some success. It might be worth reposting your query in Chat to get a bit more traffic.