They could use handbrake to transcode to something smaller. Id recommend HEVC (x265) using CPU, avoid nvenc, with 23-25 quality and the cpu speed to medium or slow. Use OPUS as the audio format to lower the bitrate for the same quality.

This typically gets 6gb files down to 600mb for me but it is incredibly slow compared to other settings.

If they want it automated then use the handbrake preset or make an ffmpeg flow in TDARR. Just give it the folder and it will optimize the videos they give it. Some setup required.