Occasionally, Warner Bros. Television will upload the first episode of Friends to their official YouTube channel as a promotional stunt for the series' anniversary. As of this writing, the full pilot is not permanently free, but you can find 5-minute clips and "Best of Season 1" compilations. Do not fall for bootleg uploads; they get DMCA'd within 24 hours.
The most reliable way to watch for free is by using introductory trials from major platforms: friends series 1 free