Is the exhaust leaking between the head and the O2 sensor, if this is happening the O2 sensor thinks the engine is running lean and will richen the mix to protect the engine. A small leak can suck in the O2 and create this problem.
It happened on my two and I couldn't hear the exhaust leak.
Here's a bit of reading for you on Lambda sensors (O2 Sensors)
The lambda may not be the problem, it could be down to bad servicing record.
Run some dipetane on a couple of tank fills and see if it improves, this will clean out alot of deposits on both the injectors and the fuel system generally.
Is the air filter clean
What condition are the plugs in, white, brown or black.
What MPG is he getting not much facts in "A full tank lasts 3 days" I often use a tank of fuel in a day but I may have driven 400 miles. Best way to calculate the fuel comsumption is to:
Fill the car and record the litres it took.
Set the trip meter to 0.
Drive till the car is empty, then refill it and record the litres, leave odo alone this time.
Drive till empty this time and record your total mileage then divide it by the total litres used and divide that figure by 5 and this will give you the current MPG.
Once you have a baseline figure you will be able to see what is helping things get better.