Sorry to hear about your mums dog, its very sad when ever we loose one of our pets.
Sudden death is uncommon in our pets. The more common reasons for suden death include:
1) Hemangiosarcoma. These are blood vessel tumors typically of the liver, spleen, or base of the heart. These tunors are fragile. When they rupture the pet bleeds internally and can pass away quickly.
2) Gastric dilitation and volvulus. This is when the stomach twists and the pet bloats. Owners will come home and find that there pet has passed away.
3) Choking on a foreign body. This is when the pet gets something stuck in their throat and can't breath.
There are many other less common reasons. Many times a veterinarian can perform a necropsy( an animal form of an autopsy) to determine the cause of death.
Hope this helps,
Thanks,
Dr Vicksman
Thanks for your answer. I initially thought it was a heart attack due to way she just collapsed, my mum doesn't want her to be 'messed around by the vet', so I guess we'll never know for sure!