1. They often help each other.或者：They frequently help each other. (each other 一般用于两个人之间，不严格时也用于三个人以上“互相”，例如口语中就不是很严格)
2. They often help one another. 或者：They frequently help one another.(one another 一般用于三个人以上“互相”)
often 和 frequently是同义词，都表示“经常”，所以可以选用其中一个就行了。
祝您进步。
