Most likely the Saudis will just either outright buy nukes from Pakistan or purchase Pakistani expertise in building their own nuclear weapons program.
It's widely believed there is an existing agreement that makes this possible.
From 2013[1]:
> Earlier this year, a senior Nato decision maker told me that he had seen intelligence reporting that nuclear weapons made in Pakistan on behalf of Saudi Arabia are now sitting ready for delivery.
> Last month Amos Yadlin, a former head of Israeli military intelligence, told a conference in Sweden that if Iran got the bomb, "the Saudis will not wait one month. They already paid for the bomb, they will go to Pakistan and bring what they need to bring."
Saudi Arabia funded the Pakistan nuclear program back in the 1970s[2].
[2] https://www.arabnews.com/node/2069831 "This was the time when Saudi Arabia opened its doors to Pakistani workers and provided financial aid to the Bhutto regime in order to thwart India’s nuclear ambitions."
