Firstly the 50 fps will not sync in the final 25 frame timeline. It will play like a slow mo and will need to be sped up 50% to sync. This will make it look like normal motion, defeating the purpose of overcranking.
To make it seem like sync when shooting at 5 fps, it will need to run 80% slower. This is because the lip sync playback was not double speed to match the 50 fps frame rate. Otherwise your estimate of 40% would be correct.
In other words, the playback rate needs to be proportional to the 5 fps. As this is 20% of your base frame rate, everything sync has to be slowed by that amount including any further playback to lip sync to.
The director made a mistake shooting the singer at 50fps without speeding up the playback x 2 to lip sync. I appreciate you can’t tell him of his basic mistakes. The client is always right – especially when they are wrong.