What is the truth?
The FODMAP hypothesis was first proposed in 2005 in this paper. The second author appears to maintain a website where she lists some foods though it's an incomplete list. Grapefruit would classified in this group if it has a high fructose level in comparison to the glucose level (frutose:glucose ratio > 1) and this likely varies on the variety. This Stanford site suggests restricting yourself to half a grapefruit if you have problems.
There is no "correct" low-FODMAP diet chart, because there is no clearly defined amount of FODMAPs that would cause problems. There are foods that contain high, moderate or low amount of FODMAPs and various authors may put the moderate ones either in the high or low category.
Also, sensitivity to FODMAPs varies greatly from person to person, so everyone needs to make a personal list appropriate for him/her -- from the trial/error experience.
Here is another list with a detailed explanation how a low-FODMAP diet works.
100 grams of grapefruit (NutritionData) contains just a little bit more fructose than glucose: 1.7 g F : 1.6 g G, so the F:G ratio is practically 1 and not problematic. Also, 100 grams of grapefruit contains only 2 grams of fiber, which also does not sound a lot.
I found the Monash University mobile app, available for both iOS and Android, to be very helpful in identifying the low and high FODMAP products. Low and high are relative terms that do not mean anything until they refer to a quantitative scale. The apps does a very good job in defining "low" and "high" based on a serving expressed in cups, grams, millilitres or typical size.
According to this app grapefruit should be avoided as 1 medium sized grapefruit contains high amounts of oligo-fructans, but half medium grapefruit contains moderate amounts of oligo-fructans, which means it might be tolerated by some individuals.