Kentucky does not require private transfers to go through an FFL, so assuming that both you and your friend are residents of Kentucky, and that you are not a prohibited person, you may legally take possession of his revolver without a NICS check. Therefore the work can occur at his house or at yours, with him present or not present.
Since neither federal law or ATF guidelines establish a monetary threshold, only a court can determine, after charges are brought by the ATF, whether your gunsmithing work constitutes a business and requires you to be a licensed FFL. If you are not charging anything for the work, then it is highly unlikely that the ATF will accuse you of being "in the business" of gunsmithing. However, there is no law that states "yes, you can perform the work."
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