RE: But one of the transactions shows proceeds of $6,000 but a cost of $70,000!

Example: If you shorted 1,000 shares of GameStop Corp (GME) at $5 on around 7/1/2020, and covered on 3/1/2021, you could need $189,000 to cover.  If you covered today, the bill would still be around $125,000.  Based on GME's financials, using the traditional pricing models, shorting at $5 might not even be that bad an idea at that time.  

Yes, as @George4Tacks suggested, ask the client -  and brace for his/her pain, that is, unless the 1099-B was wrong.  A suggestion: ask while the spouse is NOT there. 

Actually if you identify the stock in issue, you may have a clue.