Who is this mythical "greatest ally" you speak about?

Typically, such conspiracy minded people are referring to Israel, which supports Saudi Arabia as an opposing power to Iran. There is a long history of conspiracy theories of secret Israeli/Mossad control of the US government.

Is that really a conspiracy theory then? Israel unquestionably wields disproportionate lobbying power in the US.

He means Saudi Arabia; how we suck up to them because of their oil regardless of how much they do detrimental to us.

I'd like to hear him explain how does Saudi Arabia plant politicians and judges in the US...

They don't; they spend decent money on marketing to public opinion, lobbying, and buying members of The Congress.

Downvoted, this kind of conspiracy thinking needs to be backed up by facts or at least some somewhat credible sources.

Saudis have been swimming in money and investing heavily for decades. You don't need a conspiracy theory. Ownership is enough to create political power.

The "plant" statement to which you replied is framed like a general conspiracy rumour, and would benefit from references to credible sources, though.

Replace "greatest ally" with "military-industrial complex", do you still need sources?

I’d like evidence or sources for the statement that Saudi Arabia spending money and being wealthy would lead to corrupt officials (plants) in the USA. To me it sounds like an extraordinary claim.