Retrieve a new gallon of regular milk. carefully open it. Pour a bottle of hersheys chocolate syrup in. don't skimp here, just pour the whole bottle right in. Now is not the time for going cheap, just chalk it up as a cost to bear to keep your family together.
shake it up really really well, for at least 2 minutes. You could even pour it all out into a blender and fire that up to really get things moving around, and then carefully pour it back into the gallon jug.
if you sense you need to add a bit more chocolate, then separately place 3 king size Three Musketeers in the microwave, set it to high, and then once melted down, pour the liquid chocolate off the microwave plate back into the gallon jug. If you don't have 3 king size Three Musketeers handy, you can always substitute with the poor man's version and just use a family size of M&Ms — but if you do, be very careful here not to choose peanut M&Ms ---- your cover will absolutely be blown if your wife tastes peanuts in the milk afterward. Don't risk it all here with this simple but far too common misstep.
For good measure i usually will put 1/2 or 3/4 cup of pure cocoa powder in as well - i have found it helps keep that rich flavor you're going to want here.
Since you obviously want it to be cold, once you have all the ingredients back in the gallon jug, you need to super-cool the milk to seal in the flavor. Place the gallon gently into the freezer, being careful to place either ice cubes or bags of frozen vegetables all around it. Think of it like searing a steak at very high temperatures — you want to lock in the chocolate. If you don't want to risk losing any flavor at all, one thing you can also do is carefully rub Icy Hot on the outside of the plastic before placing in the freezer. just make sure to avoid near the spout as this could possible contaminate it all when poured.
After 12 to 13 minutes in the freezer, rotate and re-apply icy hot if needed. Keep in the freezer for another 5-6 minutes, and then carefully remove and set it back in the freezer.
Using whatever adhesive you have handy (scotch tape, gorilla glue, elmers, rubber cement, etc) re-seal the lid to mimic an unopened gallon of milk. Again, this seems like a small step but your wife will surely notice if this is not attended to.