If, as you are saying more than 50% of his business is to one customer and he is not keeping that customer top of his priorities, then I would say he will lose them very soon.
What have you got left then? Approx £80K of unsecured business and probably not a lot of goodwill in the area.
As it stands, from what you have told us, the business is not really worth a great deal, it sounds like it has been badly managed and he has run up large debts which he is not able to pay off. Are their any assets/equipment you could buy off him.
Friend are fine and should be helped out where possible, but don't let that draw you into a bad business decision that could cost you £,000s
I would be tempted to take him on as an employee and pay him a percentage of work that has been carried across.