Not very, it is becoming increasingly clear. Apart from possibly being a personal tragedy for him and his family it is quite possibly a factor (a major one?) in how the markets seem to be assessing the relative strengths of the UK and our fellow advanced countries' economies, who all have their own major problems to contend with. See here and here.
If what is written in the Telegraph opinion piece is based on real conversations with high-level Labour insiders (and I suspect it may be) then it is really a very worrying sign. One can imagine political opponents wanting to 'put the boot in', but when your own side starts briefing the press in this way then it is clearly alarming, and that's a mild way of putting it.