My best advice for anyone in this situation is to document everything .
I spent a few years working customer service handling orders for manufacturing company. One particularly customers was a consistent problem. This company believed one of their personnel shat gold bricks, but I realized right off that they were incompetent and used lies & intimidation to cover this up.
This person would routinely fax their orders at the end of the day (right before they would leave) without confirming that they actually sent me the files necessary to start their order, and that their orders were almost always "rush" orders with very very short turn around times. Another thing this person would do, would be to call me up, tell me they had an order and ask me what the latest day they would need to receive the order by a specific date and time. I would tell them, then they would wait well past this final submittable date, submit their order and then claim that I had promised to turn around the product by that time. Over the years, the turn around time necessary to complete their orders shrunk to impossible expectations and their customer began getting upset as my customer started blaming me personally for the delays.
The irritating part, is that whenever I some how failed to live up to this person's errors (i.e., I was unable to cover for them), they would call up my boss and complain about me. My boss only believed half their bullshit, but it was still enough to impact my career.
Unfortunately for them, one of their customers wasn't an idiot, and had remembered me when he came along to our plant for a facility inspection prior to us beginning production of their product. This customer set up a meeting between our companies and asked me point blank when I received the purchase order, when I received the files and when I delivered the product. Thankfully, I had records of the time and date of every purchase order that company had ever sent, along with records of the time and date of receipt of every file to begin production, as well as the delivery date of the product to their warehouse.
It turned out that the end customer was sending the purchase orders to our problem person up to three weeks before the problem person would send me the PO and files. The problem person would sit on the file for weeks before submitting it to their production and farming out our part to us. The problem person ended up losing their company around $2million in sales yearly when they lost their client.
We ended up being directly contracted by the end customer to continue manufacturing our part of their product.