apple-idicloudiphonemessagespassword
I gave my best friend my iCloud/Apple ID password because she forgot hers. Does that allow her to see all of my text messages that are incoming and outgoing, or only my iMessages?
Also, if my new conversations start off with my number, will she have access to them if the person I’m texting has an iPhone?
As you can see in the first screenshot, you can set a number of email addresses by which you can receive messages at on your phone. However, as the second screenshot shows, you can only have one Apple ID connected to a device at a time.
You might be thinking, "Ok, can't I just add her Apple ID email address to my phone?"
The answer is no. Any time you add an email address to your iMessage account, the email is verified with Apple. If you attempt to add an email address associated with another Apple ID, Apple will decline it and neither iPhone nor OS X will not allow you to add the email address.
Conclusion: Unfortunately, there is no way to do this with iPhone or OS X. You could do what was previously suggested and set up a secondary account on your Mac (if you have one) and link that account to her Apple ID and use Message there to view the exchanges, but to specifically answer your question, OS X does not accept more than one Apple ID for iMessage per user and iPhone does not accept more than one Apple ID for iMessage either.
I see the mostly likely cause of this being the possibility that your default sending address is set to your email address. I say this because it appears that you initiated the conversation.
You can check (and change) this by going to Settings.app -> Messages -> Send & Receive. Under "Start new conversations from:", observe whether your email address or phone number is selected.
If your email address is selected, this would be an easy explanation as to why this happened.
Otherwise, as far as I know, (and I have just went over this to verify), it is impossible to extract the email address that is associated with an iMessage phone number from just a regular iMessage "text".
Best Answer
To start off, as of August 2017, there is no way to view or send iMessages from the web. So before she could view your conversations, your friend would need to register one of her Apple devices with iMessage using your Apple ID.
When that happens, you will get a notification on all of your devices, informing you that some new device is now registered to use your Apple ID and phone number for iMessage.
Your friend will then be able to see all iMessages you send and receive. It doesn’t matter if you initiate conversations from your phone number or from your Apple ID. Not only will she be able to see everything, she can also impersonate you (by sending iMessages on your behalf, from both your phone number and Apple ID).
Regular text messages (SMS/MMS) work a bit differently. They are sent and received through your cellphone carrier, then forwarded from your iPhone to your other devices. Even if your friend registered her Apple device with iMessage using your Apple ID, you would still need to go into Settings → Messages → Text Message Forwarding and allow forwarding to her Apple device.
Currently, registering a new device for iMessage only allows that device to view new messages that are sent and received. However starting in iOS 11, with iCloud Messages, registering a new device will allow it to view all conversations synced to iCloud, past and future.
Text conversations are absolutely the least of your worries, however. Much more importantly, with your Apple ID password, your friend can login to iCloud.com and read your email, impersonate you by sending email on your behalf, use your email to change your password on other websites (and essentially login to any of your accounts, anywhere), view your photos synced to iCloud Photo Library, access your contacts and view their private information, etc.
In essence you’ve given her full access to your account, much of your private life, and private data belonging to your friends and family. In many ways, you’ve given her your identity.
Remember that arguments and falling-outs do happen, even among best friends. If/when that happens, she could also change the password on your iCloud account and steal it from you.
I strongly recommend you visit id.apple.com, and change your password immediately. Then remove any devices she may have registered with your Apple ID.
I’m just not sure how giving her your account password solves the problem where she forgot hers. What she needs to do is recover her own Apple ID.