不要让朋友失望英文可以表达为：
Don't let your friends down.
这句话的来源可能来自于人们对友谊的看法，即在朋友需要帮助或支持的时候，应该尽力去帮助或支持他们，以免让他们失望。
一些英文例句和中文翻译如下：
I know you're counting on me to help you with your project. I won't let you down.
我知道你指望我帮你做这个项目。我不会让你失望的。
Your friends are depending on you to show up at the party. Don't let them down.
你的朋友们指望你出席聚会。不要让他们失望。
You promised you would help me with my homework. Don't let me down.
你答应过帮我做作业。不要让我失望。