I would look at integrating a React Native module that implements geofencing into your app. There are quite a few options out there. This one in particular piqued my interest as it is quite simple and likely exactly what you need:
You’ll also want notifications
Steps to implement:
Does your client app only need to send it's location data when it's actually being used. and thus open on the screen Or also when it's running in the background?
With the latter there are also some third party solutions you can use. But from what i heard it's not always easy and drains the battery quickly.
Here's an earlier asked question with an answer where Traccar was used to monitor a user's location while the app was running in the background. https://forum.mendix.com/link/questions/91250
With these widgets: https://appstore.home.mendix.com/link/app/109513/ you should be able to get what you want.
Based on your (geo)location (used from your mobile device) you should be able to find out if the user is entering and predefined ‘danger zone’. Once you have determined that you can use common Mendix logic to sent out a push notification to you user.
Getting the Geolocation is default functionality of Mendix 8