RCM - Rent - Goods and Services Tax - GST

A HUF consisting of Father and son owns a commercial property. It is let out to a firm in which son is a partner (75%) share, with third party. HUF is not registered under GST. Due to financial problem in the firm, the landlord agrees to waive off rent for few months. My query is whether the firm and landlord will be treated as related parties, quantum of rent fixed can be questioned and whether RCM tax is still payable even though rent is waived off by the landlord.