Dave Baker wrote:

A leak in a heating coil somewhere inside the system causing water from the
boiler/radiator part of the system to get into the hot water part of it. The
sludge remover probably removed sludge that was actually sealing the
pinhole. Very common in the coils inside immersion tanks as they corrode
internally.

In a combi this would be a leak in the secondary heat exchanger. This
is where water from the primary circuit (ie the stuff that goes through
the radiators) "meets" the domestic hot water that comes out of the
taps. In fact that they shouldn't quite meet, they should just get
close enough either side of a piece of thin metal for the heat to pass
from one to the other.

However, as Dave says, it may be that the sludge remover was a bit too
aggressive and has corroded the metal of the heat exchanger allowing
water from the primary circuit into the DHW. This would lead to a loss
of pressure in the primary circuit as water seeped out. Is your friend
having to top the boiler up using the filling loop?

If the heat exchanger was in that state and the system was sludged up
anyway I wouldn't feel too bad about it - the heat exhanger would have
needed replacing soon. If it does need replacing it should be done as a
matter of urgency. You don't want to be doing your washing up in water
from the rads, and constantly topping up the primary circuit means that
the corrosion inhibitor is being diluted and the system is likely to
suffer from more sludge and corrosion.