Ive always thought the napkin isnt lapped until the place setting is engaged, or is left when signaling you werent going to have anything. Similarly, if a side napkin, it went to lap at first contact/use. Then again, Im no miss manners...
I think the rule is to wait for the host to put his or her napkin on their lap and then you should follow suit. Since there aint no others on the table, shes done messed up.