YES YOU COULD TAKE A BREATH!
Of course the SCUBA diver would not be breathing O2, it would be some sort of trimix. Putting that aside, if a freediver wanted to they could take a breath off the scuba divers tanks, but if they wanted to surface, depending on how many breaths they took, would have to either a) go into decompresion with that diver or b) if they only took one or two breaths, the nitrogen or helium absorbtion would not be significant enough to go into deco and could then surface slowly rembering to exhale.
Now 400 ft is a long way to do a CESA (controlled emergency swimming ascent) but it is do able, maybe not be everyone, but by someone with the fitness it takes to dive to 400 ft could. Another thing said, if they were free diving properly they would have all the nescesary precautions taken so that this wouldnt happen.