GST not recovered from client

One of my client, engaged in works contract, received payment from a registered client for commercial construction services performed in July to march but didn't issue INVOICE or charged any GST.
NOW his GST liability comes out to be RS 1lac and he is in tension.
Please guide, can we recover it from client and how? So that, client doesn't argue.
Please elaborate.

Also tell me whether interest is applicable for late payment of gst and at what rate?