Is the Farewell Circumambulation considered valid if the husband throws the pebbles on behalf of his wife while she is performing the circumambulation?

It is not permissible for someone who has appointed an agent to stone the Jamarat to perform the Farewell circumambulation (Tawaf al-Wada') until after their agent has finished stoning. This is so that the Farewell circumambulation remains the last of the Hajj rituals. If they perform the circumambulation before their agent has stoned, their circumambulation is invalid, and a sacrifice (dam) becomes obligatory upon them.