Black widow spider bite.

Hi,
A good friend of mine woke up this morning and found her cat dead in her bathroom. She said her cat was perfectly fine the night before because she let it outside and it was running around and playing then before bed she let it back in the house. It was still acting the same way, like itself. When she woke up this morning, she found it laying dead in her bathroom. She said one side of its mouth was swollen and there was a dead spider laying by the cat.

What could have caused the cat to die, with just one side of her mouth swollen? Could the spider have bit it?

I don't got the money to pay for a quick reply back but it has really upset her and I feel terribly bad because she rescued this cat when it was just a few weeks old when she found the momma cat dead. She has grown amazing attached to this cat and has had it for a little over a year.. I would really appreciate it if someone could help me with what could have caused this to her cat.

Please, please help. Thank you so much.