internal drive disk full error

I’m trying to fix my mum’s old imac which has lay unused after packing in many years ago. The internal hardrive won’t stay mounted. I have tried the disk utility first aid and it fails and says invalid index key. I have also tried fsck -fy and that fails and says disk full error. The drive is 500 gb and only has 6gb free, the majority is other storage but i don’t know if thats safe to wipe or if it could contiain photos my mum wants. I’ve booted the computer through an external drive on high sierra but i don’t really know what to do now.