It is not clear whether she is claiming she got the burrito from a roach coach low end Mexican food truck or from a more typical food truck run by someone aspiring to have a brick and mortar restaurant. According to the post below, Houlihan changed her diet in a big way and is into Shalane's "Run Fast Eat Slow" cook book. I just do not see her going to a low end food truck for a meal given the risk for food poisoning due to poor hygiene, etc.
A low end Mexican food truck isn't going to be buying a whole pig. They would just pull up to a carniceria and get some cheap cuts of pork to cook. Liver isn't used much at all in Mexican cooking. The only way it would get into cuts of pork is if the pig was cooked whole and everything was taken off the bone.
And a low end Mexican food truck isn't going to give you a receipt. I suspect that the food truck was more of a typical hipster food truck and Houlihan got a receipt by using a credit card on Square. No one running a typical food truck is going to come anywhere near a pig's liver.
https://medium.com/runners-life/how-they-train-shelby-houlihan-cdfb07273b68