there have been a lot of threads about giving gifts, so how about a thread specifically for any problems with giving gifts? what to buy, what size, where to buy, etc etc, ask and answer here, everyone helping everyone.
If you can sneak away one of her rings to bring to a jeweler (if that's what you're doing) then that would be your best bet. If you can sneak away a ring for long enough to take a piece of string or something similar and use that to measure the circumference, you can then look online to see what ring size that correlates to. Something I've done for shoe sizes (different, but could still work if you have that dynamic with somebody else) is to say "oh hey, my mom and I were discussing who has bigger feet; you or my brother. What's your shoe size?".