Temp cover on friends car - anybody had to claim?

I need to get some short term cover for my daughter on MILs new car. I have done this before but its been for a couple of hours only, this time she is taking it for ten days on a 500 mile round trip so the risk of getting it damaged is higher.

The car is only three months old and on finance so my main concern is an at fault claim or damage/theft where the insurer needs to pay out.

Has anyone experienced needing to claim on a temp cover policy and can provide feedback on any of the providers good or bad?

MIL is not keen to add her onto her own policy for whatever reason, but absolutely happy for her to take the barely used new car away as long as it gets insured for that trip.

I looked around and most are charging similar rates but my main concern is getting it fixed etc if something does happen so I'm interested in any experiences of claiming on these temporary cover policies.

Thanks