Welcome to the forums!
Your friend is correct, I'm afraid. You can only remove a follower from the tiles adjacent to a tower at the moment you place a block on the tower, and you can only remove one. If there are other followers 'within range' they stay where they are. A follower can also be placed withing range by a portal, living in constant fear that someone will place another block on the tower.
The point of closing a tower is just to protect any followers you have within its range. They can still be captured by another tower,
as can the follower on top of the tower itself. So, if you friend has used a portal to get a follower onto a farm in range of the first tower, you'll need to use another tower to take your follower off the top of the first tower, then place a block on it to capture your friend's follower.
Hope that helps. I haven't had any coffee yet, so apologies if I missed your point…