What should I do with the sum of money left by my deceased friend for his also deceased wife, knowing that she has no heirs and I don't know how to contact his children? And would the money be considered haram if I took it?

It is not permissible to benefit from these funds. Rather, they must be delivered to their owners. If that is not possible after making an effort, then it is permissible to donate them on their behalf. If you are in need, you may benefit from them to the extent of your need.