There are several possible problems with the bag causing loss of loft. Most have been mentioned already. To recap:
1) Physical damage to the shell
2) Lack of rinsing, leaving detergent or soap in the down
3) Physical damage to baffles from agitation and/or mishandling while wet
4) Incorrect or incomplete drying
I sort-of doubt that a detergent induced removal of embedded oils would be noticable as loss of loft immediatly. This would take some use/compression of the down to break the plumes. Usually the down dust will become noticable as "clumps" in the fill that are very difficult to break apart. Sometimes you may notice just a loss of loft on thicker bags. As you say, it may not meet it's temp rating.
Down is fairly resistant to heat up to boiling temps. Not so, the shell. If the shell is not damaged (zippers, pockets, and hood) this can be ignored. Use low or medium heat with frequent stops for hand shaking while drying. I use my home dryer for this, but, it is extra large capacity. Also several dryer balls. These are about the size of a tennis ball but have "porcupine quills" on them...three or four on my 0F bag, two for my 32F bags. Tennis balls are the standard, though. 4-6 of these work well.
I agree that any soap or detergent left in the bag will cause loss of loft. Try plain water on an extra wash cycle with extra rinse cycle. If the bag is over 5" thick, you may need to do this twice (6 rinses.)
Using down wash will not buy you anything. Adding soap to detergent is probably not a good idea, if there is *any* detergent left in it. If the damage by detergent has been done, there is no feasable way to restore the embedded oils to down. They are part of the structure and, if lost, cannot be replaced. The down will degrade over time, especially after the next use. But, it will be difficult to tell till it is used, or, compressed in a compression bag and shaken out. It just won't reloft. I believe that WM, perhaps others, will add more down if this is the case. Ater drying it thuroughly (as others have said, a longish process) you can return it to them for restuffing. This will add a few ounces, but, you can avoid buying a new bag. Perhaps the person responsible can help pay for it?
Down bags and stuff can certainly be washed OK. But, it takes a very mild soap. The embedded oils are washed out at EVERY washing to some degree. Even plain old water will wash out some. Body oils can stick to the embedded oils in down, also. Generally, anything that cleans body oils out of the down will also remove some of the embedded oils along with it. So, keeping the bag clean vs down degradation from body oils is a problem with down. Down wash is supposedly mild enough to remove this "loose" oil, without removing embedded oils. I often recommend about 1/4 to 1/3 as much down wash as what is recomended on the bottle, though...just enough to clean the bag, but, not enough to deep clean it. What is not cleaned will get washed next time...