Nina we have been in roughly the same situation with our letting agent. Before we moved to our current address we had a private landlord who was brilliant. If we discovered anything wrong we simply rang him. He would ask us to give him ten minutes and then within that ten minutes he would ring us back and tell us that 'Fred' or 'Jack' (or whoever) would ring us in a few minutes to book a mutually convenient time to come and do the job. The chap then rang, agreed a time - sorted.
Sadly, we had to move to be nearer our workplaces due to lack of public transport and found the ideal place but sadly it is 'managed' by an agent. We spoke to the owner and asked if she would be prepared to rent it to us privately, saving her (and us) all the ridiculous agents' fees but she explained that she is abroad a lot and needed to use an agent who could deal with any problems immediately rather than us having to wait until we could contact her - fair enough.
We were unable to use the shower for six weeks as it was seized up; we had windows that we were unable to shut because the catches were broken (good for letting the heat out!) and most recently we had a leaky roof resulting in water dripping down from above our bedroom door. On every occasion we had to continually chase the agent asking when the repairs were going to be done and each time we had the same answer - they hadn't heard back from the owner! They were waiting for the owner anyway!
Sadly, owners and agents have the upper hand which is very unfair so you have to be so careful.
One piece of advice I would give if things really get to desperation point as has happened with us in the past - NEVER EVER REFUSE TO PAY THE RENT - as if you do they can throw you out or use it against you.
I was advised by a solicitor a while ago - if needs must, pay the rent on the date it is due, then put in writing that you have paid the rent in full for the next month as it's always paid in advance.
Explain in writing that you have patiently waited for however long for the repair to be completed and that it is getting beyond a joke. Then simply say that you hope that the job will be completed by the date when the next rent payment is due, but that if it has not been rectified you are NOT refusing to pay the rent but will withhold it until the job is completed when it will be paid immediately. We had to take that action with the leaking roof and surprise surprise the roof was repaired within the fortnight.
We later had the annual inspection and I mentioned to the agent that it was just such a shame that we have to get to that point before action is taken. She tried to say that withholding rent is as bad as refusing to pay and can affect your future references but funnily enough shut up when I said we had received that advice from a solicitor and his advice would be used as mitigation if necessary!
I wish your DD all the very best as it is a real minefield and a lottery as to whether you get a good or bad agent.