Alasdair wrote:

Under the rules (Rule 2) the cheque must be dated on the actual date
of issue and under Rule 6c the cheque must be presented within 3 days
of issue else the guarantee is null and void.

So it would seem.

If she can show evidence as to when the cheque was issued in the first
place, is the guarantee worthless and is her bank likely to
acknowledge this?

That depends on how "issued" is defined. It could be argued that by
giving them an undated cheque, she has not actually issued it yet, but
has authorised them to issue the cheque on her behalf at a future date.

On the other hand, this argument seems to be defeated by Rule 6 (which
is directed specifically at encashment service providers) because it
says the cheque must be presented within 3 days of when they *received*
it. Even if they could argue that "issue" was delayed, they would still
have received it well before it was issued. Again there is a grey area
because Rule 6 applies "where a guarantee is used to support an encashment
service". Strictly, in your scenario, your friend availed herself of a
loan service, not an encashment service, and so it could be argued that
rule 6 does not apply.

This seems to be a pretty stupid way of borrowing a small amount of money.
You mention that these shops apply usurious rates of interest. Surely her
bank's overdraft rates are less usurious than the shop's, so why not just
get the money from the bank? -- Or was there never any intention to pay
it back? :-)