I'll be honest, I can't imagine how you would charge a phone in the woods.
If you slept in the woods at night, your DC lived elsewhere and and you needed to phone them, wouldn't you manage it? You'd find a way, if course you would.
You don't have to stay in the woods, the OP's ex isn't cut off from all civilisation! He just sleeps there.
There are lots of places you can go to charge a phone. Perhaps a nearby cafe might let you charge your phone for free, or if you buy a cup of tea. Perhaps there's a library nearby. Perhaps you have a half in the pub with a friend on evening and charge your phone, saving the charge for days by only turning it on when you need it.
Or perhaps, even though you sleep in the woods you still have a vehicle and can charge the phone in the car.
Perhaps you've got a wind-up mobile charger - here's a basic one for £3.96. www.gearbest.com/other-camping-gadgets/pp_425046.html
Or, maybe you have a solar power charger. Here's one for £15. www.halfords.com/workshop-tools/garage-equipment/battery-chargers-jump-starters/halfords-solar-battery-maintainer-1-5w
Or maybe you're really sorted and you store energy in 12v batteries which you charge with solar panels or wind turbines.
Or perhaps you have a petrol generator and get power off that.
Just because someone is homeless, or lives in the woods, doesn't mean that have no money, no possessions or that they lack ingenuity. In fact, to make living in the woods successful you probably need a fair amount of practical intelligence.