TBH it sounds like the inconvenience is minimal as they have given her a car to drive while they are fixing hers. Mental anguish sounds a bit OTT to me - if you can show is has caused her great mental anguish i could understand - could you give us an example of what mental anguish she has suffered, and how much money you think will compensate for that.
Ok so her wheel fell off but no-one was hurt, didn't cause a crash etc. it hasn't left her unable to drive - she is driving a rental car.
ok i can see it is inconvenient for her but surely it's only a temporary inconvenience that she will get over?
Anyway all that ^^^ is only my opinon of course, and ive never had it happen to me so what do i know? If she has legal cover on her car insurance, could she talk to them? If not, you are looking at a solicitor and once you go that way, no-one wins except them