Hi,
A pair should never be given a nest box until they have fully bonded. Your male just lost a mate and is not likely to accept a new mate right away. It would be better to have the birds in cages side by side to get used to each other rather than just put them together right away. Once they start sitting next to each other and try to interact through the cage bars, you can try them in the same cage, but without the nest box. Do not give them a nest box until the birds spend all their time together, groom each other, feed each other and then mate or show clear signs of courting.
Thank you for asking Lafeber,
Brenda