Pretty sure I lost my last startup due to my former co-founder having NPD. How did I cope? I had to walk away and lose everything I had at that time, deal with huge legal bills, and then watch him twiddle his thumbs while the startup folded. It also put an intense pressure on my marriage which ultimately also broke down.

Was a dreadful experience that I would never wish on anyone else and I am pretty certain I'll never trust anyone to co-found another startup with in the future.

Edit to add: advice for anyone in business with someone they suspect of NPD: tidy everything up, get legal advice, get away as quietly as possible, don't look back.