The trigger part of the circuit is the same for the V2 and V3 dropControllers. The only real difference between the two is that the pins moved around.
Off the top of my head the only things I can think of are:
1 – the flash pulse time may be slightly too long
2 – one of the flash guns is developing a problem.
3 – faulty 4N25s.
1. Flash pulse time
I have a very old flash gun where if the trigger signal is kept high it keeps firing. To use this with the dropController I have to reduce the trigger pulse time to around 10ms (this flash is the reason the dropController allows you to change the pulse time).
2. One of the flashes is developing an issue.
I have a set of Yonhnuo YN560-III’s and one of them starting firing slightly late which causes a ghost image as you describe.
Although you don’t say how many flash guns your wife is using I suspect it is more than one and this is the issue. 2-3ms is likely too quick for a flash to repeat fire and it does sound like you have one that is firing late.
This would not explain why V2 works and V3 doesn’t unless different flash guns are used.
Is the ghost image darker than the main image?
3. Have you checked the signal coming out of the optocouplers? This is a long shot but you never know.