The question is a bit confusing. Is the HOA member a tenant who then entered into a sublease with the current Tenant? If that's the case, the rental agreement between the HOA member and the landlord should be reviewed to ensure that such a sublease is permitted. If the Tenant is lawfully in possession, then the answer to your questions may be found in the written agreement between the HOA member and the Tenant, if one exists. If the agreement is merely that the HOA member found Tenant the apartment or is merely responsible for physically collecting rent, then the HOA member is likely not responsible for maintaining the property. However, if there is a written agreement that states otherwise, then the HOA member have some responsibility here. Generally, the landlord is the one that is responsible for ensuring that the property is in a habitatble condition. So, without knowing more, the landlord rather than the HOA member, should be contacted regarding the mold. Please note that the landlord should be given an opportunity to remove the mold from the unit before anyone starts suing.
No, the tenant has no standing to sue the HOA directly. The tenant must resolve the issue with the landlord. Only the person on title to the property has voting rights, and only such owner can vote and deal with the HOA on HOA issues. (Civil Code §1363.03.)