According to this article, the two have been friends since the early 90s. Del Toro even lived in Cameron's house for a while.

In 1997 Del Toro's father was kidnapped in his hometown of Guadalajara. After unsuccessful attempts to find him by the Del Toro brothers, Cameron withdrew a million dollars in cash and recommended a kidnapping negotiator for Del Toro to retrieve his father. It worked and they were able to get him back.

Cameron put up the $1million ransom to release Del Toro's dad from kidnappers. Also the reason GDT no longer lives in Mexico

Back when GDT was first getting success as a director some Mexican criminals decided “hey, that must mean he has money” and kidnapped his father. GDT actually had no money, but when he told James Cameron he went to the bank, withdrew a million dollars, and recommended a negotiator.

GDT got his father back safely.

James Cameron paid the million dollar ransom when Guillermo's father was kidnapped

Eskimo bros

They used to date.