Ironically, this song isn't the best evidence of RW lip-synching.
Based on what is flapping around on the web, the story goes along the lines that on part of the 2007 (DSOTM) tour, RW wasn't well for some gigs - I have no idea if this is true. In turn, the "rumour" continues that because it affected his voice, backing tracks were played to "flesh out" his own "live" voice. Given that RW's voice is not the strongest vocally (that is surely indisputable even for the most ardent RW / Floyd fan), it's plausible. If Roger was still "singing" live, to his own backing track, it's arguable this isn't actually lip synching; it's a sort of personal karaoke.
For my money (no pun intended), having seen several examples of the alleged vocal backing track / lip-synching on YouTube, I tend towards the camp that reckons the sing-along explanation is true. It's particularly noticeable on the material from The Final Cut. Even allowing for the fact that YouTube vids can be manipulated wholesale to create the appearance of something that didn't happen, there are too many different examples for it to be dismissed as "mischief". Interestingly, I haven't been able to find any public comment from RW on the issue which, if the sing-along / synching allegation was NOT true, might be seen as a bit surprising. Either that, or the enigmatic silence is seen as merely adding to the charm and mystery of the event....
I suppose another point that arises is "Does it matter?" I guess that comes down to what is being sold v audience expectations v "Was it a good show in any case?".
To directly answer your question Pierre, I have not witnessed it first hand.