In the future, finger tight is fine. I think you are going to have to clean your coil housing, and your tank, to (maybe) get things working again. If you have one of those "bottle-unscrew" devices to help open stubborn lids, that might work, I use one on my tank from time to time. You can also try rubber bands, to help let you get a grip on the tank, so it comes apart without breaking. I'm a little worried about juice seeping down from the tank into the battery, if it's frozen on there, but first you need to get it open. You can try putting it in the freezer for a bit-- the metal may shrink while the glass won't, which may allow you to take it apart. If all that fails, maybe time for a brick and mortar store to help see if they can get it apart... You really need to, in order to a) clean it and b) make sure no e-juice has gotten down into the battery, which MAY be causing your problems.
I hope you have a decent e-cig store where the employees can be of help. You may end up having to warranty it, depending on what happens if/when you get it apart. I'd make sure, if that happens a) don't give it to your friend to use again and b) make sure you watch a few youtube videos about the device (it should be an easy search) to make sure that you are setting everything up correctly with no leaking, etc.
Good luck!
Anna