If they actually went through with this (I'm somewhat doubtful) then either the Soviets and their client in Kabul just got a massive windfall, or some enterprising reporter is probably going to write up a story about a US helicopter destroyed by a stinger given to the Afghans.
Well, the first is sort of true. Basically the Afganistan war now has a competitor for the foriegn fighters who went to help the mujaheen. Of course if having more arab nations occupied by foriegn powers in somehow more upsetting to the Arab world you could see an increase in the numbers of such fighters, perhaps to the point that they don't see a decrease.
NOw the possiblity of a stinger being used against an American craft is a real one, that was discussed OTL.
Much more likely here of course.
Such an event could likley lead to a shut off of stingers being sent to the Afganis, but if the Sovs are heavily funding the anti-americans in Iran, it might be more cost effective to hurt the Soviets as much as possible despite the occasional backflow.
Indeed this senerio is a mess. Very dangerous.
There's been no consideration of what impact this would have on the region.