How would you break it to someone that their DPS is awful?

[Tiggy]

I would let them be. No one is going to be great at everything.

We had a FC member who was quite honestly as awful as the OP's scenario. Every party would noticeably be hindered by their presence and it was detectable without any parsers. We let it be too and in the end you know what happened? He was always left behind on the sidelines because people started avoiding him for any and all content. This passive aggressive solution isn't a solution. It's running away and not dealing with it and in my experience it made things worse and made that player feel ostracized. Be direct and kind but don't ignore it.