So why didn't Chris stop Emily? Instead of trying to get help, as you're supposed to do for a friend who informs you of a suicide plan, Chris aides in the ending of Emily's life. The reason that Chris did this is very complex. For part of the time while Emily was planning her own death, Chris could have been in denial.
Even though he knew that his girlfriend was suicidal, he could have not realized the full extent of the problem, or not wanted to accept it.
My theory, and the one that seems most plausible due to the characters' relationship is that by helping Emily kill herself, Chris actually thought he was helping her. He was setting her free from a life she hated living, and he didn't want to betray her trust by telling someone of her wishes. Chris did what he thought was best for Emily, just as he had done his whole life.
People often make mistakes, think that they can solve problems that are in reality too big for them to handle. Most of the time, this ends up in a bad situation, but in Chris's case, it ended up in the death of the person he loved most.