In reply to:
Der Max hat's angestiftet (Max abetted me).
A friend of mine was recently in the States and while speaking to a friend in German, someone interrupted and told them it was rude to speak foreign languages in company (butting in on conversations, it seems is widely accepted)
My little foray into German was inspired by another thread about overused words, such as "actually", "like", and, in German, "oder", "natürlich" and "grundsätzlich".
I just thought I would have a little fun and throw another overused English construction into the mix, while testing the waters to see if "eigentlich" is as abused as "actually." As for the execrable quality of my German, at least it's no worse than the quality of my English! The use of the present tense was deliberate. I hear "I know that already, actually" quite often, at least as often as "I knew that already" - bad English transformed into very bad German.
As to your friend being told that it was rude to use another language in the presence of those who don't speak it, I grew up being taught that also. However, I don't always agree. I think that, for example, if one had something important to say, and clarity of communication was necessary, then why not use one's mother tongue. I guess the suspicion is that if someone starts talking in a "foreign" tongue, that person must be saying something rude! 
