Ask your friend these three questions;
1. What “toxins” is this fruit diet supposed to clean you of?
2. How (specifically) does this fruit diet cleanse you of these toxins?
3. Is there any evidence to support this?
Chances are, they won’t be able to answer the first question. They may cite some random quack, but be skeptical of that person as well. Check their credentials and fact-check their claims. If what they’re saying seems to go against the majority of professionals in that field, they’re likely a charlatan motivated by their own interests.
If you don’t want to intrude too much, I’d advise them to consult with a registered dietitian before adopting such a restrictive diet. Fruits tend to be a poor source of calcium, iodine, iron, zinc, protein, and many more nutrients. Fruitarian diets can work for some, but they need to be carefully planned and followed. Something that a dietitian can help to design. That being said, I doubt any dietitian would see the need to cut out vegetables, legumes, beans, nuts, etc. unless they have some rare medical condition. I imagine this would be incredibly wasteful too.
As with many of the over-the-counter detox supplements and “cleanse” diet plans, be very skeptical. There’s little evidence any of these things remove “toxins” from the body.
https://nccih.nih.gov/health/detoxes-cleanses
Some of the diet regimens are effective at helping you loose weight, like some fruit-based diets, but that’s to be expected when all of what you’re eating is not very calorically dense.
We should take great care to distance these extremely fringe diets as far from veganism as we can, in the exact same way that we should distance racism from veganism as far as we can. I suspect that occasionally, people suffering from diet-related obstacles (eating disorders, orthorexia, body image, insecurity etc.) seek solace in these fringe diets. They delude themselves, or are deluded by others, into thinking that veganism will somehow be a “cure” to their problems.
Moving forward, if you don’t want to outright tell your friend not to be stupid, I’d just warn them to consult a dietitian before making such a change. The dietitian will probably set them straight for you.