There are many theories, facts and myths when it comes to consuming food. Well-intended people give poor dietary advice and sometimes poor habits become entrenched and passed-down through generations. There are also many factors when it comes to food consumption - quality of food, time of day, general health, quantity and combinations, etc - so that what works for some does not work for others and misconceptions develop.
As a general rule, eating fruit and meat at the same meal can cause digestive difficulties as different enzymes are used to ensure best digestion and they are interfered with when other enzymes fight them. Different foods, raw or cooked - and different cooking processes such as steaming, roasting and so on - mean that foods require different lengths of time to digest causing some foods to remain in the stomach or digestive system longer than they need to if others are ingested simultaneously. Fruit is a raw food mostly and is quickly digested so meat digestion can obviously slow this process and cause fruit to start fermenting (sometimes causing bloating, gas, etc)
Fruit contains sugars which are broken down easily and meat which is a protein takes much longer to break down so there are recommendations that the fruit should either be eaten first or sometime after the meat to allow the meat to begin the digestive process first.
If a friend recommends avoiding fruit and meat at the same meal, his or her reasons will be linked to digestion so making educated decisions would work best rather than acting on hearsay as being too prescriptive about what to eat together can lead to its own set of problems and can even lead to important foods being excluded altogether.
My response would be to applaud him or her on an understanding of the processes of digestion but to advise against too strict an adherence to this recommendation on a large scale. Perhaps avoid acidic fruits with proteins and do not eat fruits straight after a meal. Wait about 30 to 45 minutes by which time the meat will be digesting (or digested) appropriately.
Food obsessions are very much a part of the modern lifestyle and could become the problem ultimately. There are other combinations - mainly with starches - that cause more digestion difficulties and enjoyment of food can be seriously hampered if a friend stresses too much. A healthy diet with the right quantities of fruit and veg, protein and carbohydrates with some good fats thrown in is the best policy. The friend should only worry about food combinations such as fruit and meat if digestive discomfort is experienced.