Unfortunately, any number of things can cause the upper to come apart (aka, Go-Kaboom), anything from bad metallurgy to faulty ammunition to a bore obstruction. Causes could range from sub-par component quality, to lack of maintenance leading to dangerous overpressures when firing, to bad ammunition. It is not unknown for firearms to go kaboom on factory ammo, and there have been a number of recalls on factory ammo that have been found to have overcharges.... guess how they learned of the overcharges? Yep, consumer reports of kabooms.
Sadly, unless more information can be gleaned about incidents like these, the simplest (and most common) conclusion is to blame bad parts. This, however, is not always the case. While it is usually not done in a deliberate attempt to cast aspersion upon (fill in the blank) brand, that is usually the net result.
I would urge that potential buyers do their research, which includes searching for testimonials (both good and bad), and asking for recommendations (on forums such as these), as the OP has done, and to try their best to separate the the brand-bashing from the nuggets of truth. Not easy, I admit. Especially in regards to firearms (witness the endless bashing of Glocks, Hi-Points, hollow-point vs ball, 1911 vs Glock, and the neverending 9mm vs .45 debates and name-calling).
I personally know nothing about Del-Tons, had never even heard the brand until last week, from the OP, and reserve my comments until I have heard first-hand information from an owner of same. It would behoove others to do the same.
EvilD, I mean no offense. Your friend probably hates the brand with every fiber of his being, but it would be very helpful for us if you could get more information from him, and share this information. Thanks.