INPUT TAX CREDIT RELATED

As per section 16(2)(3) of the CGST act 2017 , Supplier must pay GST collected from you to Govt account to avail Input Tax credit .

Yes , this is big flaw in Return Filing system in GST & it cannot be flawless unless the matching system the core base of GST is implemented (GSTr 1,2,3) .

Yes ofcourse your client should worry because in term of Section 155 of CGSt act the burden of proving ITC claimed is on the Recepeint. so if the Supplier is not filing his GSTR 3b , that means he is not depositing the tax collected . so ultimately the Recepeint who is liable to deposit with Interest.

If supplier is not filing GSTR-3B means he is not paying the liability which he is collecting from your client. In this case if your client has paid to the supplier and he has a proof of it then there is no need to worry

Yes, if you have the invoice with you,no need to worry.

as per section 16 of CGST Act , one of the condition is to claim ITC is Supplier has deposited the tax & if he is not Filing GSTR 3B that means he has not deposited the tax , hence need to be reversed.

Note : Kindly refer the rule 36(4) of CGST act also.