The friends daughter Ellen does not look like her father who he knows so she must look like her mother.
So many people don't get the answer straight off the back because after reading the riddle they think because he never met his friends wife he can't say "you look just like your mother" because he doesn't know what she looks like.
What does a riddle like this suggest about the role of bias in the ambiguous language?
There can be many different interpretations and several possible meanings. It all depends on the person reading the riddle. one person might think one this and another person thinks something totally different.
There are ways we can avoid this type of ambiguity by adding words that will remove ambiguity