Omgosh - I think you're right and he did have a panic attack - they need to socialize that poor baby asap or he really may end up having so much trouble that he really does stop breathing all together - that's really sad he's THAT moritified over leaving his house....
The only other thing I can think of is maybe he DOES have a medical condition - because that kind of panting and gasping is NOT normal for ANY sized dog....
You sound like you have a good head on your shoulders and maybe you could try to work with him more and talk her into taking him in for a vet check ? He sounds petrified of LIFE.
GOOD LUCK !! Poor little guy - your post was hard to read...I feel so bad for him.