My friend installed a cleaner app after seeing a virus warning, and now the phone keeps showing alerts about viruses, storage and memory. There are several other cleaner apps on it too.
He has precious photos that may only be on the phone, so I don't want to risk losing them. What should I try first?
Back up the photos before doing anything drastic. Copy them to a computer, a compatible USB drive or a cloud account your friend can access. Check that the copies actually open, and make sure you have the right account details before relying on a cloud backup.
Don't factory reset the phone until those photos are safely copied and checked. A reset erases data stored on the device.
Those warnings could be advertising from the cleaner apps rather than proof of a virus. Uninstall the cleaner he downloaded and the other cleanup apps he doesn't need, then restart the phone and see what happens.
If the pop-ups continue, check other recently installed apps. Utility apps such as PDF readers, flashlight apps and QR code readers can also be sources of unwanted ads. Remove unfamiliar or unnecessary ones one at a time so you can see whether the alerts stop.
_OFFLINE_for_a_moment:
There's no need to remove every PDF reader or utility just because of its category. Start with apps he doesn't recognise or no longer uses.
Also check browser notifications. A website can get permission to send alerts that look like virus warnings, even when no virus has been detected.
In the browser's settings, look for site permissions or notifications and remove or block unfamiliar sites. The exact menu depends on the browser. Don't tap the warning or call a number shown in it.
If the alerts keep coming after the suspicious apps and browser permissions are dealt with, a factory reset is an option, not the first step. Back up and verify the photos first, then make sure your friend knows the account credentials needed to set the phone up again.
If you can't get the photos copied or aren't comfortable troubleshooting, a reputable repair shop may be able to help. Ask about the cost and explain that preserving the photos is the priority before agreeing to a reset.
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++narrow~escape!clause:
It may be worth checking for photos in messaging and download folders too, not just the camera folder.