I see both sides. I would usually say that Boy1 pays, but to be fair, if Boy 2 had his phone out in the playground and if phones aren't allowed in the playground, then Boy1 wouldn't have expected someone to have a phone out and could have kicked the ball at his friend with the full expectation that he's suitably ready to take the hit.
In my children's school, phones are allowed to be carried to use after school but aren't allowed to be used during school hours and most of the pupils stick to this to avoid phone confiscation. If this was the case, Boy1 isn't responsible at all, as they're just boys aping around in the playground and he can't reasonably have forseen that Boy2 had taken his phone out.
If for eg, Boy2 had his phone in his pocket, and him and boy2 were rolling around playfighting and the phone was crushed I don't suppose most people would think that that would be Boy1's responsibility. It would be (imo obviously) Boy2 who should realise he has a phone to protect and Boy2 who should take reasonable steps to look after it.
I entirely agree that people don't take enough responsibility for the things their children do but in this case, I can't see why Boy2 shouldn't bear responsibility for having his phone out in an area where others play football. I once stood talking to another mum where boys were playing football and I took a ball really hard to the side of my head (much to my children's amusement and enjoyment). I didn't for one second think it was the fault of the mortified boy whose parent dragged him over to me to apologise, rather I acknowledged it as an accident and completely my fault for standing where I was standing.
I wouldn't have expected him to replace my phone if it had been damaged, except in the case of a malicious aim, which his wasn't.
I think unless it was deliberate, which OP says it wasn't, that Boy2 pays and it would be a nice gesture if Boy1 contributes. But if Boy1 can't really afford to, I really don't think it's his fault and I speak as someone whose daughters phone was broken at school in similar circumstances.
We paid and although I felt a little rattled at the stupidity of the other party, I could see it wasn't really her call.