You could ask one of your friends whether?

You could ask one of your friends whether they were going to have a birhtday party for the boy who is you best friend and tell them that you'd buy the gift that you believe he must enjoy.
你可以问你的一个朋友他们是否打算为一个男孩开生日聚会，他是你最好的朋友，并且告诉他们你将会买一个你认为能使他高兴的礼物.

the boy who is you best friend

who is you best friend 是the boy的定语

tell them that you'd buy the gift that you believe he must enjoy.

that you'd buy the gift that you believe he must enjoy 是 tell的宾语从句
that you believe he must enjoy 是the gift 的定语

for 是为谁而举办的生日party；the boy后的who是定语从句；that you believe he must enjoy也是定语从句，你认为他一定会喜欢（的礼物）；
and tell them，是整个并列句子的第二个谓语部分。整个是并列句，主干是You could ask ....and tell ....，其中的从句就是分别引出的。

你可以问一下你其中的一个朋友是否他们在参加他们最好的朋友的生日聚会时告诉过生日的朋友你买的生日礼物时你认为他肯定会喜欢的礼物。that you believe he must enjoy是定语从句修饰 the gift

这段话的意思是:
你可以问问你的一个朋友他们是否将要为你的最好的一个男孩朋友举办party,并且告诉他们你想给他买一个你认为他一定喜欢的礼物.
(其中"for the boy who is you best friend an