How do you fix safari when it crashes due to a third party application?

If a 3rd party app is causing a default app like Safari to crash, try to set back the phone to factory settings first. Then test Safari, if it works, install the 3rd party app and try again. If Safari crashes, you can't use the 3rd party app if you want to use Safari as well.

The personal data on your phone could be in danger if the software on your phone got jailbroken or modified in another way.

Unauthorized modification of iOS can cause security vulnerabilities, instability, shortened battery life, and other issues

Use iTunes to restore your iOS device to factory settings - Apple Support