I think the point is that 'not capable of digesting' is an inaccurate description of the issue. Most likely it would just take some time to integrate meat back into their diet... I've made much smaller changes to my diet which caused discomfort, but it lasted a week or two, and my body was fine with it. Likely this friend's transition would be more uncomfortable, and lengthy, but I can't imagine that if for some reason they HAD to eat meat again, their body would completely reject all meat indefinitely.