In Lightroom CC Classic v8.2 I have an image that I flipped horizontally. Then I 'open in Photoshop' (version 20.0.3) and it is upside down. If I rorate 180 degrees, then it shows correctly.
It's safer to shoot raw + JPEG. That way if you have some image files with clipped shadows or highlights you can use the raw file with Nikon Capture NX-D to recover them. Then export the raw files to JPEG and rotate using any other photo editor that has that capability. Make sure to use a high JPEG Quality setting like 100% in the Capture export settings since you'll be resaving the file after the flip (i.e. compression artifacts) When you resave it the 2nd time you can use a lower quality setting like 80% and over-write the current file. Send those files to your friend. Once completed you can delete the raw files.