"Went south" is an idiom that means to decline or go wrong. There are a number of synonyms that can be used to convey the same meaning. Some options include "went downhill," "went awry," "went off track," "went off the rails," "went sour," "went amiss," and "went astray." Each of these phrases can provide a slightly different nuance to the idea of something going wrong. For example, "went sour" might imply that something started out well before turning bad, while "went off the rails" suggests a complete breakdown. Using these synonyms can help to add variety and interest to your writing.