Just to clarify, when you see they are "a leader," are they currently THE leader of the guild? As it is, current ranks only allow for others to either invite, or not invite, kick, or not kick, etc. There cannot be multiple leaders.
That being said, see if they have permission to invite. If their guild rank does not allow this, have someone who does have permission to invite try to so so.
Now, I see that you said that he is the only one in it, with the invite feature greyed out, Im going to have to assume that he is the only one online? Because if he were the only *member* in that guild, there should be no problem inviting, as he would supposedly be the leader (owner) of the guild.
EDIT:
Everyday, two hours before siege and an hour after siege ends, you cannot invite players to guild
Disregard everything I said. Quote above is your answer.