Your friend has encountered the iPadOS15 “shelf” - a new multi-tasking feature of iPadOS15. On the basis of your attached image, your friend currently has many open Mail Windows…
The shelf is applicable to many Apps - including Mail and Safari.
In this instance, tapping the Mail App icon summons the shelf view - which will show you a thumbnail image of all the open Windows/instances of the Mail App along the bottom of the screen. Where multiple instances of an App are running, a simple tap of a chosen thumbnail will instantly switch to that window/view. Tapping anywhere else on the screen causes the animated collapse of the shelf view.
For example, if you have three running instances of Mail, tapping the Mail icon will open the shelf with four thumbnails - one thumbnail for each of the three running instances - plus another (with a “+’) with which you can open a New Window (instance) of Mail. Simply tapping a thumbnail will switch directly to that running Window/instance. Swiping a thumbnail off the top of the screen will close the associated Mail Window/instance.
This article from Appleinsider has a useful explanation of this new feature along with many of the updates to multitasking:
https://appleinsider.com/articles/21/08/12/how-to-master-multitasking-on-ipad-and-ipad-pro-in-ipados-15
The iPad User Guide is also a rich source of frequently overlooked information. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892
It is perhaps worth noting that the shelf is a fundamental element of the revised User Interface - and cannot be disabled.