Just go to Eterna city, and go into the Pokemon centre. Talk to some lady (i think she has green hair), and she give you the friendship checker app on your Poketch.
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In Pokemon Diamond the Friendship Checker (also called the Happiness Checker) app is #6.
The friendship app or frenship checker is given to you by a girl on the first floor of the Eterna city Pokemon center.
from a woman in eterna city's Pokemon city
You can find the app in the Pokemon Center in Eterna City. Talk to one of the girls.
In Eterna City's Pokemon center. Talk to a woman in there and she will give you the poketch app- freindship checker.