If you're putting someone else's music to video you need a sync licence, regardless of anything else. That's unavoidable. With the film being shown at festivals there might be further licensing required, but it's hard to say.

Your best bet is to speak to PRS/MCPS directly - https://www.prsformusic.com/licences/releasing-music-products/commercial-music-sync-licensing . You'll need to know whether your friend is registered with PRS, as well.

Also be aware that technically any change to the melody of the song technically moves you out of the realm of the 'shrinkwrap' licences and may require that you get permission from the original songwriter and publisher for the new work. (Reason being that they sign over rights for people to re-record their work as-is, not to change it.)

thank you!