Weather app showing wrong temperatures

Are you sure that the Temperature Unit is set to your preference. You can check by going to Settings > General > Language and Region > Temperature Unit. You can change your preference there if it was originally set incorrectly. If that does not work, you could try Restarting your phone by following these steps: https://support.apple.com/en-sg/HT201559 and if that does not work you could try Hard Resetting your device.

Here’s how to Hard Reset your iPhone:

• iPad models that don't have a Home button: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Power button until the device restarts.
• iPhone 8 or later: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Side button until you see the Apple logo.
• iPhone 7, iPhone 7 Plus, and iPod touch (7th generation): Press and hold both the Top (or Side) button and the Volume Down buttons until you see the Apple logo.
• iPad with Home button, iPhone 6s or earlier, and iPod touch (6th generation) or earlier: Press and hold both the Home and the Top (or Side) buttons until you see the Apple logo.

If all of the above steps do not work, you could try contacting Apple Support via The Apple Support App on your iPhone: https://apps.apple.com/sg/app/apple-support/id1130498044

Hope this was of help!

Given the amount you state, is it possible that her weather app is showing a different temperature scale, such as Celsius instead of Fahrenheit?