Your friend left his son!!!!! Jesus I don't understand parents who leave children. Either male or female. However the courts do not care about morality, only legal issues that all assets belong equally to each side. The pension is NOT HIS, it is THEIRS. The house is NOT HERS, it is THEIRS. That is called marriage. Irrelevant who paid for the mortgage. Irrelevant who might have saved or spent money more. It's called marriage.
Anyway, you add up all the assets, house equity, all pensions which get allocated an official CETV value. You then divide by 2. Then negotiations start.
Your friend has left his son. He is living with his mum. Maintenance is NOT relevant to this. The son needs somewhere to live. It is reasonable to say the mum needs more to cover housing costs for the son, but the income levels are taken into account of mother and father. The split of the 100% assets might not be 50/50, it is based on the needs of each family member.
Negotiations might lead, if the house and pension are roughly equal value, to the house going to the mum and son and the pension going to the father. It sounds like this may be the solution in this situation, but a bit of adjustment might be needed, so one of the sides might have to pay a cash sum to the other to get to the percentage negotiated. Both sides need to have a reality check that they either work it out now or they go to court and the judge will divide 40/60 or 60/40 or whatever and either side will likely pay their own legal costs. If you represent yourself in court, of course the costs are zero if you are the defendant. I had to represent myself, simply because my husband who left 4 children wanted 55% of the assets. He spent 25k trying to get 55% of the assets. The judge awarded him 37% of the assets in the end and I paid no court costs. Court is inevitable if either or both sides refuse to accept the reality that EVERYTHING IS OWNED JOINTLY and that the person left with the greater burden for children / lesser earning power gets a greater percentage of the joint assets.