Can one have leftovers from ones wife when she becomes a nidda being that the leftovers is from the day before she became a nidda?
It is somthing really mutar but yet still leftovers.
There are a number of ways that what are colloquially called leftovers can avoid the status of "shiraei ochel shelah" (remnants of her food). According to the packet my wife got in kallah classes, if eaten on a different plate, the food may be eaten. Similarly, if the wife is not present, the food may be eaten. Both of these are far easier to accomplish if these are leftovers from yesterday.
Like Dov F, I am tempted to compare this to אברי בשר נחירה שהכניסו ישראל עמהן לארץ (Chullin 17a, and see Rosh ad loc.). During the 40 years in the desert, meat killed in a certain way (nechira) was permitted. Upon crossing into Israel, that method of slaughter was no longer acceptable as shechita. Was meat left over from before they crossed the Jordan acceptable, or now that new rules of shechita apply, is the old meat prohibited? This is a very obscure question of halakha seemingly with practical application beyond a few days in history. However, the Rosh says it theoretically applies to any time a new rule is created. Since yesterday the food was permitted (pre-nidda), in theory the ruling about the leftover nechira meat would apply.
From Family Purity by Rabbi Fishel Jacobs
I don't have either of those books or I would look it up. The phrasing "permitted to finish" makes we wonder if it refers only to a case where both are [already] eating from the same plate. As opposed to taking her plate and eating from it.
As far as leftovers from yesterday goes, as soon as she leave the room, or the food is moved to a different plate it becomes mutar.