iMessage not working on iPhone without sim
I can iMessage from my Wifi only iPad, so it can be done. I have a feeling it has something to do with the sending or receiving address. I would check the Messages setting to make sure it is set to her Apple ID/email address. I would also try, instead of her sending it to your phone number, having her send it to your Apple ID/email address, as that is more universally recognized. It would be interesting to know if she can iMessage other iPhones as well.