No. Greece cannot leave the Eurozone. It may be good for Greece but Italy, Portugal, Spain would follow soon after. The greatest concern for stability in the EU is that there would be a domino effect with countries leaving the EU and Greece seems like the obvious starting point.
Moreover if Greece did leave. Then Russia would probably be the most likely country to offer financial backing seeing as Greece would have to find some form of financial support. However I would almost certainly expect a US response to this where they would either attempt to outbid Russia to be Greece's financial backer (unlikely) or use media to make Putin seem like the bad guy (very likely).