Just come right out and say it. Wouldn't you like to take it past the friendship level with her?
It'd be a bummer to put a damper on a really fun thing but maybe you should slow it down a bit if you are starting to feel those feelings where you want to be more than friends. Especially if she isn't in a solid marriage and could give into temptation. You know it's wrong so you might have to distance yourself to stay out of their way. Part of the fun is that mysterious chemistry because you don't know where it will take you. Who even knows how she feels about it, maybe she really does see you as just a friend. You could ask her but it could get awkward after that. Wouldn't you rather it be awkward than to be the other guy?
It's a tough call but you know how you feel and what your intentions are.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.