Let user quit web app to native navigation app and get him coming back during a process

(Please edit the title if you have a better one)

Imagine a mobile web app, where the user needs at some point in a process to start a turn by turn navigation and then perform an action in the web app once s/he reached his/her destination.

I see two possibilities:

1. Develop a turn by turn navigation by myself into the web app

For this I would go with mapbox's API's.

Advantages:

  • User stays in the app
  • Flow is not interrupted
  • User will more likely perform the needed actions once reaching the destination

Disadvantages:

  • Need to develop the navigation system
  • More expensive (mapbox plans)
  • Even if the custom navigation could have some good quality it will probably not beat googles implementation

2. Open google maps from within the web app

It is actually possible to either just navigate to the google maps website passing some coordinates (android will then automatically open google maps app) or open the app from within the browser for IOS users. But in the other direction I think it's not possible to navigate back automatically to the web app (browser) when closing a native app.

Advantages:

  • No need to develop nav myself
  • No costs for navigation itself
  • Probably better navigation system

Disadvantages:

  • Styles do not correspond to app
  • User is pulled out the app
  • User may not (come back) finish process as intended
  • One more click on native app (maps) for starting the navigation
  • What if the user does not have the google maps app on his device (I'm not sure if navigation is possible from the google maps website on mobile)?

So I probably missed some more points but I think the decision is already quite difficult to make.

My question is, from a UX point of view, is it acceptable (probably not) to let the user leave the app and hope he will come back or is there another solution which I don't see?

A solution would be to send a push notification and remember the user to finish the process but push notifications are not really supported on IOS web.

Also one argument why the user could come back, is that he already did a certain amount of actions during the process before starting the navigation and he would probably be willing to finish it, but there is still the possibility he would not.

Any help would be greatly appreciated.