Nixon acknowledged and worked with the Communist Chinese government so the two of them could prevent Soviet influence from spreading to Asia, yet when the Soviets invaded Afghanistan they did nothing as far as i am aware.
They did actually, the US plans to supply Afghan Mujaheddin involved using Soviet designed weapons to maintain plausible deniability (any weapons found on Afghans can simply said to be captured from the Soviets) and for that purpose they needed a large supplier of Soviet arms. While many ex-Soviet clients such as Egypt had the capacity most were either incapable of supplying the amount needed or did not want to supply too much weapons as to anger the Soviets if uncovered. China however, were both willing and able to sell weapons to the CIA at large profit margins who in turn sent them to the Mujaheddin as well as turning some weapons to the Afghans directly.
The majority of weapons/arms supplied to the Afghans before 1984 (and possibly after) were Chinese made, making the PRC the "supplier-in-chief" of the Afghan guerrillas against the Soviet Union.
Source: Ghost Wars: The Secret History of the CIA, Afghanistan and Bin Laden