The real question is, "Am I afraid of offending her or am I afraid of embarrassing myself?" If she said she was ok then what are you afraid of? I assume you're going to be wearing more then just the diaper and since thats underwear, so what's the big deal?
You have nothing to worry about if she is fine with you wearing. Furthermore, people tend not to notice that you are wearing a diaper underneath your clothing in public. In fact, I have worn thicker diapers out in public (Abena M4, ConfiDry 24/7) on numerous occasions, and no one noticed anything. The initial fear of a stranger noticing your diaper in public is completely overblown. The general public isn't nearly as observant of diapers as we happen to be.
If she's ok with it I would say go for it and as izzy said most people are not as observant as us and unless they are looking they probably won't notice and if they do they probably won't say anything because they have they're own problems or things to do just don't do anything stupid and you should be just fine and will probably have lots of fun.