Was this person by any chance previously sent an invite link with editing permissions, and add the tree to their account, did they?
Because if so you're going to need to have this person remove the tree from their account, then send them an invite with the new permissions.
The reason the tree being on the person's account prevents the new invite link from taking affect is because when the person open the link the site see that the person has access already and just opens the tree, same as if they just opened it wthout the link.
