Was it intended that the joint owner have a beneficial interest in the
account?
If the joint ownership was for convenience only, the surviving joint
owner may hold the account on constructive trusts for the estate, in
which case any IHT is payable out of the estate.
However, if the joint owner has a beneficial interest and the account
passes to them by survivorship, the account is aggregable property and
the IHT on the account is payable by the surviving joint owner
If the wording of the will is, for example “ I Declare that all inheritance tax payable by reason of my death shall be paid out of my residuary estate” then it would seem to me that the tax attributable to share in the joint account would be payable out of the estate.