My tandem-partner says короче. I asked her to tell me the meaning, but she refused to teach me 'bad Russian' (I don't know if she was kidding, though).
I've seen короче in Lingvo but that source doesn't provide the expected meaning.
Just a filler word. Associated with being cool and kinda redneck nowadays. A colloquial contraction from an already colloquial "короче говоря" (in a nutshell, to make a long story short)
Some people are just too used to insert it in the beginning of the sentence, which is grammatically correct but still looks like a verbal tic if you love it too much.
It means something like "without going into detail," usually after an attempt was made to give the longer version or, literally, "in short". For many it is a parasite word, getting into their every other sentence. It's not as bad as "так сказать" but similar.
I have a Canadian friend who injects "так сказать" into almost every sentence. I witnessed him talking to his Canadian friends and putting it into English sentences. Your friend is right, treat it as if it wasn't spoken at all.