A customer came in and purchased a large Cashier's Check to buy a "summer home". Questions were asked and even followed up with a phone call. She assured us that it was legit and she knew what she was doing. A week later she calls and says she was scammed. Do we need to follow up with a SAR? The only subject we know of is our customer.

What was the amount of the check?

over $90,000

The customer should have contact information of who she was dealing with, even if it's not legitimate or vague information (ie. Daniel with Tech Support Inc.). Your SAR obligation depends upon the dollar amount.

A $90,000 scam demands a SAR with as much info on the purported seller as you can get from your customer. Your customer should be identified in the narrative so that interested law enforcement can contact her. I'm guessing here that your customer is elderly. If so, you may have a reporting obligation to Elder Services or similar office in your state.