Discussions on the internet are failing more and more. We should work on fixing that
Asking the why of something is always a good way to begin a conversation with the intend of changing someones mind. You must first understand where are they coming from. For all I know, they will always buy the same jar of peanut butter because thats the peanut butter that their now deceased mom used to make their sandwiches or they simply find the taste fascinating.
Once that you understand why do they do this or that, which is rarely simple, you may then make suggestions. By making suggestions I do not mean that you bluntly show stats, data or evidence and tell them they are wrong, but rather you make suggestions into the air, progressively showing new information and different point of views.
For example, lets say your friend always bets on the Cowboys, irrelevant or how they have performed. In the case that, say Patriots, are doing better and one could potentially profit off them with a higher degree of probability, any reasonsable being should drop the Cowboys and back the Packers. However, irrational behaviours such as undying loylaty or more complicated situations will prevent this decision from even being considered. This is precisely the point when you ask why.
Why do you bet on the Cowboys? Why do you even bet to begin with? Why this why that. Guide them into questioning themselves on their beliefs. Show them the path with easy questions.
Once your friend is questioning wether he should bet to begin with, you may show him that the Patriots are doing better, starting with simple suggestions and moving on into presenting more structural data. This process may result on your friend betting for another team, thinking and trying something different. You did not force him into doing anything, you just asked him, indirectly, to step out of his ways and try something new