My dad hit me when I was a teenager - once. When we were kids we occassionally got spanked, but that was during the time when spanking your kids was an acceptable and normal form of discipline. The time when I was a teenager was the result of an argument that kept escalating, and like any teenager worth her salt, I knew exactly how to push his buttons, and push them I did! He backhanded me across the face, and his ring caught me near the eye, giving me a black eye. He was immediatly horrified, even though I knew I had seriously earned that smack in the face.
I had fun for a week telling all my friends that "My daddy gave me a black eye". I can only imagine now how horrified he must have been. He was NOT an abusive man, he had no history of violence, but he was not necessarily a patient man and he got pushed over the edge and lost control. People are human and they make mistakes. Sometimes really bad ones.
The answer to your friend's dilemma lies in the circumstances and the history. Spanking a kid hard enough to leave a bruise sounds pretty over the edge, but are there mitigating factors?
A good family counselor is the best person to really assess the situation. If I had one piece of advise for this woman is, put off making any decisions until you calm down, have some time to think about it, and see a good counselor.*
* - If child is in danger, ignore the above and get thee gone - immediately.
ETA - I just read the orginal poster's response. Dr. Genie's response is spot on. Sounds like the husband needs individual counseling, and the family needs family counseling. One thing that stands out though - why did the little son not say anthing about being hit with a broom handle? That seems unusual to me. Most little kids I've known would run crying to mom about something like that.