Buddy of mine noticed the plastic plug that covers the 'spring caging bolt hole' had opened up. He stuck his finger in the hole and felt broken spring pieces and guessed that when the spring broke it blew open the cover. Is this possible?

your friend gets dummy of the year award if that brake chamber had sprung/split in two
while his finger was inside and he was right next to it it could have killed him. to all new drivers this hole is for caging only, not fingers
the spring is huge and a few little pieces doesnt mean theirs still not thousands of pounds of pressure on it.

The rubber plug can come out any time. You should try to keep it on as it keeps crap out of the chamber. I would recommend only visually checking the spring instead of sticking your finger inside they usually have on air leak when the spring breaks because it punctures the diphram.

I've had several springs fail but never punctured a diaphram? I just noticed that the spring brakes were not working well and found the rod not pushing out. This seems to happen most often in the spring when the salt has got into the spring and the warmer weather activates the corrosion. I pull the brakes when I'm still rolling slowly anytime I'm not hitting a dock so's I can know if they are holding or not - it's hilly around here and I often stop on an incline and I really want to know the brakes will hold!

I got in the habit of looking at the brake chambers while doing my pretrip. If you are on the driver side of the truck, it is fairly easy to see the passenger side brake chambers with a good flashlight. If the brake chamber is hanging at an odd angle (usually parallel to the ground), I get a mechanic to look at it. I was shown a blown brake chamber on someone elses truck that was hanging parallel to the ground. It really looked strange compared to the others.
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It is a whole lot easier for mechanics to fix in the yard than out on the road. Drivers where I work do not check squat before driving the truck but that is another story.

If it was blown, there would be a MASSIVE air leak. Usually not a good idea to stick your finger in there, but it won't "blow apart" unless you take the big ring off.

The best way to check is to hit the chamber with your fist(pinky side), sort of like if you're mad at someone and you want to hit a table or something. Don't use your knuckles! If it's broken, you'll hear a rattle.

If it is broken, get it fixed ASAP. It WILL eventually puncture the diaphragm,.