hang about - he's signed a contract (i assume for 12 or 18 months) and missed a number of payments, so on what grounds will he be able to get out of that contract on the basis it is they (O2) who are in breach of that contract ?
am sure there will be some small print in that contract that says somethings along the lines of "will will cease service provision if you dont pay"
if you've signed a contract stick to the terms of it by paying, or if you want out of it pay their cancellation fee and get a PAYG phone