Is it permissible for the husband to seize his dying wife's money and deprive her legal heirs of it?

The husband should be advised and reminded of the sinfulness of his action and of Allah's punishment for the oppressors.

If that does not work, the matter should be referred to the Sharia court or Islamic centers.

If only a non-Sharia court is available, and it is permissible for the heirs to reclaim their right from it, then there is no harm in resorting to it, provided only the right is taken.

If none of that works, then the appointment is before Allah, where the oppressed will be compensated from the good deeds of the oppressor, or the sins of the oppressed will be cast upon the oppressor.