It really depends on the expenses for the child. In my experience, a court ordered maintenance will be one monthly sum which will already include school expenses or childcare costs, if any, and medical and birthdays/christmas, etc.
Keeping in mind that each parent is equally responsible, €850 x 2 = €1700 a month to support one child: that seems terribly expensive to me unless there are creche fees involved.
The practical thing to do is for your friend to ask for a list of the child's (reasonable) expenses, including everything, split the total in half and agree a new monthly payment. If he feels some expenses are unreasonable, he can negotiate them. That way, he knows what to pay every month and he shouldn't get hit for extra sums during the year. It's better to try and work it out between them than have to go to court, where everything then seems to get nasty.