I think any politician worth his/her salt would be able to handle an enthusiastic constituent that wants to discuss the issues. In fact, it might be welcome. I'm sure once you get to an office at that high a level that you don't get many chances to talk to members of the general public for any meaningful length of time. The PM may actually enjoy hearing from this person, even if they disagree on some issues.
I think that as long as the guest is polite, there's nothing incorrect about bringing up controversial issues to a politician if given the opportunity.