How to get the Wi-Fi option to turn on after being grayed out?

My friend bought a used iPhone 4 that works perfectly fine, except the option to turn on Wi-Fi is grayed out. We turned it off & on, reset the Network settings multiple times, updated it, restored it. Airplane isn't turned on. It's out of warranty, so Apple can't fix it. Is there anything else we can do?

If she docent have unlimited Internet on her plan and she used all of it instead of using Wi-Fi it blocks itself out

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