How do I share a friend's location with the app ,,FIND MY" so he or she can't figure it out?

The standard Apple location-tracking features will not provide what you request. That omission is intentional. Apple location tracking is opt-in.

Per Apple: Checklist: If you want to make sure no one else can see your location - Apple Support

Some further details to ponder: With various jurisdictions, this activity might be considered a prosecutable offense akin to stalking, or domestic abuse, or otherwise. Laws here can vary.

Absent any potential civil or criminal repercussions, I’d expect the use of what can be known as “stalker-ware” to end the friendship and to end any existing relationship. Minimally, the use of “stalker-ware” will erode existing trust.

So you want to track a person without their knowledge and without knowing you are tracking them?