As a direct result of the so-called ‘Brezhnev doctrine’, the USSR asserted its “right and duty” to go to war in foreign countries “if and when an existing socialist regime was threatened.” This accounts for the increased overseas military, political, and economic support being given at this time to pro-Marxist régimes in Nicaragua, Angola, Ethiopia, Mozambique, Yemen, etc. e-ir-info
If Yemen was a Marxist regime, then presumably it supported Soviet action - because the way to spot the freedom arising from Socialism & Communism is to recognize the slavish adherence to ideology. Freedom, after all, is slavery, and the class interest of the proletariat classes is best recognized in their inability to express any opinions differing from their overlords.
If I were researching for a model UN presentation, I'd make sure that the Yemen in question was aligned with the Soviet axis of the Marxist Church, and that their ideology wasn't contaminated by any schismatic forces.
The PDRY, with support from the Soviet Union, Cuba, and East Germany, responded by invading the north using 3 regular divisions and a Tactical Air Force regiment. Wikipedia
If the PDRY was deploying troops Soviet support, then it is likely that the PDRY adhered closely to whatever claptrap was emerging from Soviet apparatchik at the time. If the Soviets stated that their invasion of Afghanistan was necessary to defend communism from the extensive Afghani Capitalist Bourgeoisie, then the Yemeni would nod and smile and produce twenty-seven eight-by-ten color glossy photographs with circles and arrows and a paragraph on the back of each one illustrating the evils of the Capitalist pig dogs controlling the Afghani civilization. Truth, after all, is a lie; only Soviets tell truth; everything else in the world is merely the ramblings of counter-revolutionary propaganda specialists.
One other note - I don't recall the Yemeni order of battle, but I doubt that the Yemeni did anything in Afghanistan. I don't think Yemen had the ability to deploy forces overseas, and unless they trained with the Soviets, they'd be more of a hindrance than a help. Yemen's stance was probably just to (verbally) support the Soviet defense of the Afghan Socialist Republic [Sic]. A quick google search supports my hypothesis that Yemeni troops don't deploy outside of Yemen.