A husband died before performing hajj. May the hajj be performed on his behalf (haji badal)?

Yes. If the deceased had already become obliged to perform hajj (being able) but had not done so, according to the Shafi'i school the cost of the proxy hajj must be taken from the estate before it is divided among the heirs.
A woman from Juhaynah asked the Prophet ﷺ about her mother, who had vowed to perform hajj but died before doing so; he replied: perform hajj for her; if your mother had a debt, would you not pay it? Pay the debt of Allah, for Allah is more deserving of being repaid (Sahih al-Bukhari 1852). An important condition: the person performing the proxy hajj must already have performed hajj for themselves. In Malaysia, Lembaga Tabung Haji and recognised agencies arrange proxy hajj; make sure you use a trusted channel. If the deceased was not yet obliged, proxy hajj may still be performed as a contribution by the heirs. See Managing an estate.