tomvyn wrote:3) the actual answer to your question can be - try it without sugar at all - put it in before drinking.. this kind of thing you described is caused by sugar (why - don't ask me ;o).
I second that. Syrup is basically sugar and water; if your friend oversweetens their tea, or doesn't stir the sugar in well, they're basically making tea + syrup.
An airtight seal may help (to prevent the syrup from going slimy, at least), but better might be going with southern-style sweet tea: a bit of black tea cooked in a pot of water, added to a pitcher with some sugar, then diluted with water. The mixing the extra water into the pitcher further mixes the sugar in as well, preventing that syruppy separation.