Step 1:
Find the other person's activity from which you want to create a Strava segment. Click the three dots, then select "Export GPX."
Step 2:
Take the resulting GPX file and upload it to GOTOES here: https://gotoes.org/gotoes/strava/Add_Timestamps_To_GPX.php
The reason you need to use GOTOES is because the file you get directly from Strava does not have timestamps. This means that it will not look like a "real" activity to Strava. By adding timestamps, you will be creating a temporary "real" activity in your own account. You do not need to change any of the settings on the page above, just choose the file and then click the big blue "Upload" button to go to the next page. The default settings will work fine for this purpose.
Step 3:
On the next page, scroll down to the bottom. If you have a GOTOES account, you can check the box that says, "Send to Strava". If you don't have a GOTOES account, you can leave that box unchecked, and a file will just download to your computer. At this point, you should click the big blue button that says, "Click Here to Process The File Using Above Parameters"
Step 4:
At this point, if you selected the "Send to Strava," you can now go to your Strava account and make a segment from someone else's track!
Make sure you delete this temporary track once you have made the segment. I usually bookmark the segment so I don't lose it as it is being populated. If you did not check the "Send to Strava" box, that is OK; just manually upload the file that downloaded to your computer and then continue making a segment from someone else's track as if it were your own.