Denim and/or corduroy will be your friend. It's heavy, durable, washable, and cheap if purchased at a thrift store. I made a quilt for my daughter out of heavy cotton denim (one side) and flannel (other side) squares, and it has the kind of weight your client is asking for.
While mine was a rag quilt, yours could be something else. What I learned is that if you put denim on one side of a quilt, something softer (like flannel or regular cotton fabric) on the other, with cotton batting in between, you get a nice heavy quilt.