You might want to compare the pop-up blocking settings between your computer and your friend's. Since we don't know the friend's browser/version (e.g.IE11) or even your browser/version, it might be simplest if you just Google (if need be) how to check those settings.
I logged into several Hotmail (and Outlook.com) accounts yesterday and none of them gave me a popup. However, I was using IE11 with the popup blocker set to the highest level. You might be dealing with malware. It might be best to run Malwarebytes and let it get rid of whatever it finds. If you opt to install Malwarebytes, uncheck the trial offer near the end of the installation process.
See this post for more info.
Let Malwarebytes do the quick scan first and get rid of whatever it finds. Restart the computer and do the full scan (and get rid of whatever it finds).
If you happen to read this next part about TeamViewer and decide to not use that, please read thru
all of the info anyway.
One way to help a friend with things like this is to...
...have them install
TeamViewer (TV)*
...set TV to start with Windows
...pick a good password**
...set the remote computer to not sleep
...leave the computer on 24/7
(until the issue is fixed)
*You can install TV at this point and take control of the remote computer. That will let you do the other steps for your friend.
**It does not have to be super hard to guess. TV is very secure in several way to prevent other computers from connecting.
Then you can work on the remote computer while the owner is not using it. Once you have established one remote control session via TV, your computer becomes a trusted computer - this allows you to chat with the friend via the TV app. You can ask the friend if it is a good time for you to take control and try whatever actions get suggested here.
You do not need to setup an account with TV. Your friend does not need to setup an account with TV. But accounts are free if you want one. Accounts allow you to save the connection info (including password) so that a double click on the remote computer of interest establishes a remote control session. If you opt to set that up, then protect your computer really well and don't tell TV to automatically sign you into your account.
Another nice thing about using TV - it lets you restart the remote computer in the safe mode (with networking). You can still connect and remotely control
your friend's computer while in the safe mode (with networking) to see if the popup happens.
I could list other security related scans to run against your friend's computer - but let's just try the things listed above first.
