Abuse of position of legal deputy - any advice please?

[Mopsadaisy]

Am hopeful that someone may have experienced a similar situation and will be able to advise on the best course of action and possible outcomes.
Bil was made legal deputy for my Mil by the Court of Protection and since her death it transpires that there are thousands of pounds of missing funds.
We know the Office of legal guardian will not investigate as she has already died. BIL has forwarded bank statements so we know that there has been inappropriate spending but do not have all the information re: cheque payments. He has not offered any explanation, apologised or suggested a plan for repayment and is now saying he is too upset to respond to emails. Can anyone offer advice on possible course of action?