Academic here, with an interest in appeals because one of mine has recently managed to fail his PhD, so I am learning all about how it works, at least in our place.
It won’t be irretrievable at this stage. The important thing is to find out what the due process is and follow it to the letter. Grounds for appeal for us include extenuating circumstances which the markers would have taken into account had they been aware. It sounds like you’ve tried this but you can (and in my view should) challenge their rejection of the extenuating circumstances statement because it wasn’t submitted on the day the result was published. That’s pretty unreasonable unless they have made it very clear beforehand. It’s also stupid because it can take time to assemble such a statement and ours have to be supported by paperwork (eg doctors’ letters) which can take time to assemble. My lad has 20 working days to appeal his decision.
From what you have said, there may also be grounds around inadequate supervision/poor advice but be very careful about this one. It is very easy to get her tutor/mentor on the defensive but the tutor can and should be useful so you don’t want to alienate them unless you have to.
I would assemble all the arguments (write them down) and take them to the mentor as a first step. Ask the mentor if it needs to go higher up the food chain at this stage- course director, head of department…. Keep it civil and make notes of any meetings or phone calls, either at the time or immediately afterwards.
Also, do not involve lawyers at this stage. That will guarantee the university gets very defensive and sticks to the letter of the rules, whereas they can and ususally will be flexible if you give them the opportunity.
Good luck and get back to me if you need anything else