It is always difficult to know the answer to this question. Having lunch is not strictly forbidden. You just have to be able to tell if it may appear that the person who paid for your meal tried to influence you. It is really a judgment call, on a case-by-case basis.
It also depends on your power as an MP. If you are simply receiving information over a meal, you can probably take the steps a member would normally take and that would be acceptable.
I recently wrote a long section about this in the December 2013 Paradis Report. I made a distinction between those who can do something to advance a cause and those who cannot. However, we must always examine each and every case. Most of the time, members of Parliament have far more latitude than parliamentary secretaries or ministers.