For crying out loud - how hard is it?
You putt out and say "4" or whatever you had.
If the other person doesn't get the hint and reciprocate by announcing his score you say "6 right?" when they putt out.
If they say "No, 5", you count them out aloud. You say "Oh...I thought you left one in the bunker..." or " but it was three putts wasn't it?"
He'll get the message. You don't need apps or any technology
Alternatively, get a card and say "OK, 4 for me and 6 for...