What should I do, as I vowed to slaughter two sheep for my friend's mother if she recovered, but she passed away? Is it permissible to donate their value?

A conditional vow is prohibited, but if it is an act of devotion, it must be fulfilled when the condition upon which it was made is met. It would go to the heirs of your friend's mother if it was a vow made for her, and donating its value is not permissible. However, if it was a gift, then it is not obligatory to fulfill it, and its owner can expiate it with an oath atonement.