In "He caused her to do it" you have an infinitive phrase with its own
subject (her).  The phrase is the direct object of "caused."  The whole
thing would be diagrammed in the usual direct object slot on the main
line of the traditional diagram.  There are a number of verbs that take
this kind of infinitive, but right now I can think only of of "ask,"
"want," "order," and "allow."  "Let" and "help" take, as direct objects,
infinitive phrases with subjects, but these phrases are without the "to"
marker.
 
Michael Kischner
North Seattle Community College