It's impossible to tell if "they" is the colleague or the friend. You need to rely on context - would this colleague be likely to tell you about his or her own situation if you didn't otherwise know him (her)? Most likely, that person is telling you about your friend - but there is nothing grammatically to make that more likely.
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Anonymous"I have spoken to a colleague of my friend, who has told me that they will not be at work tomorrow"When you speak to Person X, then Person X can tell you something. When you speak to a colleague (or father, or cousin, or classmate) of Person X
I'd like to know if the relative pronoun 'who' is referring to 'a colleague', or 'my friend'.
AnonymousSo its a question of context rather than structure?Yes. In this case. Often the structure can disambiguate the antecedent, but in this case it doesn't. You have to use additional knowledge about the world. The structure alone won't help.
Grammar Geek I misread the question.Hee, hee! You should be so lucky to have a nickel for every time I've done the same thing! Not a problem. Look at the bright side: You provided "bonus information"!