Always worth asking. And yes the maintenance is for general everyday stuff. Larger items such as school trips or bedroom furniture can be negotiated as extras.
Bear in mind though that it could be argued that uniform is general everyday stuff. Children wear it rather than other clothes which you'd have to buy more of instead. Also you can get generic uniform very cheaply at tesco/asda/woolworths, this makes it ona par with normal clothing costs really. Also schools tend to have 2nd hand uniform available which can cut on cost and hand me downs are always an option.
But if the child stays overnight with ex she might suggest that as maintenance is reduced for that overnight he should contribute the 2/7ths or howevermany nights it get's reduced by towards this cost as it's something he would be expected to provide for the time child is with him.
It all depends on how she broaches it and how reasonable they both are. Ex's tend to be ex's for a reason so these things are of course not as straight forward as they should be.
The other option is to approach grandparents. My ex's parents made a significant contribution to my son's uniform costs. It enabled me to buy him a couple of items (jumper, pe shirt and 2 shirts) with the official school logo on which are 3 times the price of the generic ones out there. The rest I bought as generic stuff.
In reality I didn't think there was much point in buying school logo shirts. Proper white collar shirts look much smarter, are cheaper, and if you put a jumper over then you don't see the logo anyhow!
Your friend should be putting money aside all year for uniform. I regularly buy long/short sleeved shirts and trousers/shorts out of season when they are cheaper. Then I put them aside for next summer/winter.
It's about budgeting. Yes it can seem like a huge expense. But if you look at it as a years worth of clothes, it doesn't work out that much per week averaged out.
Gilly