"Run out on" is a common phrasal verb that means to abandon or leave someone without warning or explanation. There are many synonyms for this phrase that can convey the same meaning, including "forsake," "desert," "abandon," "betray," "leave high and dry," and "walk away from." Each of these alternatives emphasizes the sudden and unexpected nature of the act of leaving someone behind. Using a synonym can help to add variety to your writing and create a more nuanced understanding of the complex emotions involved in such a betrayal. Ultimately, the exact synonym used will vary depending on the context and tone of the writing.