I've had good luck with YCBM. You can get an account for free. You can pay and get more features, but here's what my free account looks like: https://musicmanumit.youcanbook.me/ -- you can set up codes and stuff, but I'm not sure if you can do that in the free account. She should be able to invite you and you can use the ics file. I'm not sure about the other way around.