Not likely unsafe from a pathogenic bacteria standpoint assuming she is planning on cooking it to at least 160 F. However, shepard's pie is often made from leftover cooked ingredients. This means spoilage bacteria may have had a day or more to begin to grow BEFORE the 4-5 day wait in the cooler. At the very least, quality will be compromised.
It all comes down to if the food was properly handled and stored prior to putting the pie together. The only potential food borne illness I can think of might be staph intoxication, because that toxin is very heat stable. This would require several hours of staph growth in the danger zone prior to refrigeration.
If a friend invited me to dinner and said "here, I made this shepard's pie five days ago, try some", I think I would surely pass