The Beis Halevi records the following interpretation:
שנה באבות (ה:כג), "מה בין תלמידיו של אברהם אבינו לתלמידיו של בלעם הרשע?" ושאלו המפרשים, מדוע לא כתוב, "מה בין אברהם לבלעם?" אלא, שבלעם עצמו יכול להיות נביא גדול, אולי אפי' גדול כמשה. ביוצר השיטה קשה להבחין לפעמים מהותו, אך בתלמידים כבר רואים מי הרב שלהם.
Some have thought that the following explanation ought to apply to Mendelssohn. Even if in his life he appeared to be an observant Jew, one can judge waht sort of person he really was by looking at his disciples (Most notably the notorious David Friedlander. Interestingly, Graetz refers to as Mendelssohn's Ape (Popular History V. 5) which presumably means to say that he lacked a real understanding into his mentors philosophy.)
This explanation has two difficulties. As Lion of Zion comments, the term disciple is not entirely correct, "most of his so-called disciples matured independently of his tutelage and some spent minimal time in berlin altogether."
The second is that if one is going to blame Mendelssohn for David Friedlanders tergiversation ;) then one would have to blame the Noda B' Yehuda for Aaron Choriner's actions, which is quite plainly ridiculous.