To use friendship with a third party as a device to make an argument, It cheapens and demeans both the relationship and the writer.

Ever read the scandal when V.P. Thomas Jefferson demeaned President Washington to his correspondents? Recall at that time the Prez and Veep were the first and second vote getter, not a paired-ticket. The Veep disagreed with the Prez and impugned Washington's mental abilities. What was gained by Jefferson that could not be gained by honorable means and reasoned argument? If honor and reason are inadequate, why pursue a goal for which these attributes are insufficient.

In the end, Washington got a personal denial from Jefferson that any such letter existed (not knowing that Washington had it) and further, that he (Jefferson did not believe that. Washington accepted that and never revealed the letter or its source to Jefferson.