for glass to glass contact problems think it's also been suggested to try proprietary penetrating oils such as WD40 - and seem to remember someone said warm olive oil............ both of which might be aided by lateral pressure on the stopper.
logically I'd have thought of suggesting warming the body of the article ..... with the thought that some heat might create expansion thus helping to free the stopper. However, perhaps the body might not take kindly to that method - I'm not sure.