Yes, this is an issue and you should worry.
One license purchase can be used on 1 domain at a time, which has to be setup in the XenForo customer area.
You should NOT be using the license. This could likely lead to your friends license being terminated/suspended.
There are many free options if you need to wait for revenue. Use one of them.
See the link here and filter by "free": https://www.theadminzone.com/threads/tblos-the-big-list-of-scripts-software.151872/