OK this "friend" of mine says he can join and plans on joining the Royal Marine Corp. for the United Kingdom becuase he is English by decent. Can someone clarify this

I think he needs to be a citizen of the UK or commonwealth countries. Pretty sure that they don't care if great granddaddy came from Liverpool. They would consider your buddy a US citizen

03 is correct, unless he is a citizen, he will not be joining the Royal Marines. It is possible he could apply and be granted citizenship if one of his parents were citizens. If it is a grandparent, the UK does not consider this a "direct descendent" so citizenship is not granted to grandchildren.

Good cause he would be a disgrace to the millitary name. Youd have to know him to understand how ridiculosly stupid he is.