Is the husband's abandonment of the house without paying the rent for the last six months considered consuming others' property unlawfully, or is it his right in exchange for what the landlady took without his consent, given that she had put a lower rent amount in the agreed-upon contract?

Your husband must pay the agreed-upon rent, which is 350 euros. What the landlady did by writing an untrue rent amount without his knowledge is a mistake for which she bears the consequences. If this results in the loss of a state grant or harm to your husband, he has the right to complain about her to the responsible authorities. Whatever rent remains due from him after that shall be compensated by the benefit he missed out on.