How can I concatenate 2 variables to access array values?
I think you need to try and access the array that user.foo would have.
So your code would be something like:

 {% for user in plugin.data.users %}
  ...  
  {{ attribute(user.foo, loop.index0)|raw|nl2br }}
  ...
 {% endfor %}


I haven't tested this though, just going by what I'm sure I remember doing once...