If the supplementary notes to his lectures are located on a locked website (i.e.: locked by a username and password), then an electronic reference wouldn’t be of any help since, like you said, it would be impossible for the other readers to access information pointed out by your footnote/endnote.
When you made your initial request for information, I thought you meant that your professor made it possible for anyone to download his lectures – including people that do not have access to Wien’s Campus Intranet Server (i.e.: through an open blog / public website).
Since an electronic reference can’t be used because of accessibility reasons and since his lectures weren’t published, the following footnote/endnote reference should be included to your paper:
Markus Ebner et al., “Psychologie für Physiotherapiestudenten”. Supplementary notes given on [include exact date when the notes were made available online or in the classroom] by Professor [include your professor’s name and academic title here] for his lectures within [include the name of the University here] [include the code or the title of the course here] course.