This is theft and it is abuse of a vulnerable adult. It is a police matter.
You should refer it to the police without further ado.
Can I clarify anything for you?
The more evidence that you present to the police the better. Because of the amounts of money involved here, they are probably going to look at it more seriously than if it was just a few thousand pounds.
He may be married to her but the phrase “with all my worldly goods I the endow” doesn’t extend to the contents of a bank account which are in her sole name.
If you can get bank statements showing the money being transferred, that would really help you.
If he refuses to hand statements over to the police, they can get a court order to make the bank produce them.