Do not feed fermented syrup to the bees. It could cause belly-aches and dysentery. If it is fermented enough, the byproduct which is alcohol is toxic/poison to bees. It could kill them.
It certainly does not have to be perfect, as they are going to add some yeasts to it and ferment it themselves a bit right before they use it. However, I would suggest that you leave it up to the bees if they want it as is and if they can handle it or not. They know what is best for themselves. Do this by giving them choices. Put the questionable feed out in a feeder in an area 15 to 20 feet from the hive. Let them go get it if they want it. If they take it, great. If they do not touch it, you know your answer. Also consider to put out in the area some fresh syrup. Continue to feed whichever they take up with the most vigor.
Do NOT bottle feed the fermented syrup directly into the hive. That gives the bees no choice but to take it or to cleanup whatever drips/leaks onto their heads and into their hive. They have no way to cleanup other than with their tongues. Like you cleaning your kitchen floor or the bathroom with your tongue, having to do so in all likelyhood will make them ill.
If you do not have the open feed option and you must put the feed directly on or into the hive, then mix up fresh syrup.
Hope that helps!