Telling my friend its time

My best freind has a very old and tired labrador.  As a young dog, he was a great friend to me, and loyal dog to her.  Now I fee terrible, becasue he just isnt healthy and is really old.  It seems like he is in pain and I want to tell her its time to put him to sleep.  Can anyone help suggest a way for me to bring up the subject without causing her a lot of pain?

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