I'm so very sorry to hear about this. It must of been quite a shock for all concerned.

This is a pretty common scenario and the majority of cases that I have dealt with like this are down to a misunderstanding of roles. This, of course, may not be the case here but you may be able to connect with it.
What I usually see is the first dog being treated as more important than or as an equal to some family members (sleeping on the bed, pulling on the lead, defending the territory etc. - there are so many subtle little things that creep into everyday life that can give a dog this message about their staus). When a second dog is introduced it tends to become the centre of attention and the first dog can feel that it's position in the pack is threatened. If the first dog has felt that it is Alpha and suddenly all the pack are paying more attention to a second dog, giving it privilages that should be reserved for the top dog ( getting on the furniture, being greeted first etc.) then the first dog will try to regain control. The problem really comes when we start to look at domestic dogs as Alpha animals. They are simply not up to the job of leading a pack of humans and are not equiped to deal with leadership in the human world. This is plain to the dog as it's pack goes about their daily business and it can cause the dog considerable stress. A strong, confident Alpha rarely resorts to physical means in order to subdue lower ranking individuals - an underconfident, worried and stressed leader will do so more often.
Leaning over the first dog is a threatening stance and the dog, under stress and trying to maintain its position, snaps to try and ward off this dominant approach.
We truly underestimate the potential damage that our pets can do with even a quick snap.

If this sounds like the possible scenario for your friend then all is not lost. It will need the help of a behaviourist who can visit and help the family "on the spot" but the situation is salvagable if that is something they wish to do.