Damaged a friends car. The total value of the car is $150.Damage costs are $1000.Will it be a write off?

I accidently backed into my friends car when it was parked in my drive way. I hit the back corner bumper and misplaced the bumper and left a dint in it. Now the problem is that I originally sold the car (that I hit) to my friend for $150.It is an old Datsun Stanza and in poor condition. He didnt spend too much getting it ready for rego. The quote for repairs is over $1000 because there is a huge amount of rust in the boot. If they try and bang out the dint the whole boot is likely to collapse. The car is so rusty I doubt it will pass rego this year. If it wasnt for the rust the quote would be much less. If I go to my third party insurer Im concerned they will consider the car a write-off as the damage far exceeds the value of the car. Any advice? Anyone been in a similar position? I want to do what is right by my friend but I cannot afford to fix the dent and the rust (they have to replace and reinforce parts of the boot) out of my own pocket. That is what I have insurance for.