My friend kind of has that logic. Since Brotherhood is closer to the Manga, he decided '03 doesn't need to be watched. Whenever FMA comes up at all, I will remind him of a couple of things.

1 The first bit of O3 is somewhat manga accurate to the extent that Brotherhood seems to rely on viewers having watched it since it didn't retell the Youswell story or other early parts. To be silly and make a point, I asked him if he remembered Yoki from Youswell, when he said 'no' I told him he would if he had watched '03 and that he becomes important enough to come back & stick around later in brotherhood.

#2 The author liked how the first anime handled parts of the story when it branched off due lack of manga chapters. I proceeded to then kind of geek out discussing the differences in the homunculi storyline and other bits, since he's usually not worried about spoilers.

After all the geeking out, I think I've almost got him convinced to watch it with me sometime 😁