Okay, what? That makes absolutely no sense. If they assume and know you you speak and understand the language, then they should speak that language. It'd be like going to England and having a person trying to speak like an American despite the fact that aside from slang, pronunciation, and such, British English is the same as American English. Or a mainland Chinese person speaking horrible Cantonese when Mandarin is fine in Hong Kong.
I've had that happen to me. Tons of foreign exchange students and such from Asia, right? Freaking everyone assumes I speak Mandarin Chinese. I can understand Cantonese, but I can't understand Mandarin. This only happened with a Chinese guy who was in a German class I tried out and swapped and a girl who was lost. Man, I can't wait for Koreans, Japanese, Thai, and every other Asian to come up and ask me in their own language. Oh, and I can't wait for that one day some dude who wants to practice walk up and try to start a conversation with me in Khmer.
See, this is why I want to learn Jamaican Patois or something like a Scottish accent to mess with people. Someone walks up speaking in Korean and I say in Portuguese I have no idea what you're saying.
Your friend doesn't happen to be named Johnny and has a goatee?