Sell down river means to betray or abandon someone, particularly in a manner that is harmful to them. However, there are various antonyms for this particular phrase. One antonym could be to support or defend someone in their time of need, making sure they are not left stranded. Another antonym could be to uplift or encourage someone, pushing them forward to succeed rather than leaving them behind. Additionally, working with someone rather than against them could be considered an antonym to sell down river. Ultimately, the antonyms to sell down river are all centered on helping, supporting, and empowering others, rather than abandoning or harming them.