Personally, I'm a nazi when it comes to the condition of my newest guitar. I don't even wear a belt when I play for fear of scratching the back of it, so there is no way I would wear a ring while playing it. My older electric, however, had quite a few battle scars when I got it so I wouldn't have a problem adding a few more.

I actually used to wear a thumb ring on my right hand and hammer the strings while using a wah pedal. I got some really cool sounds with that.

Anyway, I agree with the guys that say it's a personal choice. If it's uncomfortable when playing or you don't want to scratch your guitar, take it off. If she has a problem with that, she has some insecurity issues to deal with.