You need to clarify is this a claim for unpaid rent by your husbands father or for damage caused by your husband to the property they are 2 completely different claims.
This is an attempt to make your husband liable for the alleged debts of his father.
Whether your husband is a beneficiary or not does not make him responsible these appear attempts to make him responsible.
Your husband has no responsibility to the landlord personally
The debt, if any , would fall to be paid from your husbands father's estate assuming anyone is actually administering the estate either as an executor of a Will made by the father or as a result of the intestacy ( failure to make a Will).
A defence should be filed stating in bullet point that the defendant is not responsible for the debt. That the defendant is not the personal representative /administrator of his father. That the claimant could have checked this by doing a search at the probate registry.