This has always been a concern here. I remember one member quitting because she felt like everyone was criticising her weight loss choices, when they were just incredibly concerned that she was suffering an eating disorder and needed help.
I think it is ethical to point out if someone is making a dangerous choice. A lot of the time they're doing it because they don't know the risks involved. Society preaches that weight loss is as simple as 'calories in vs. calories out' so people can start dramatically cutting calories and increasing exercise not realising that they are effectively starving themselves because their net calories are so dangerously low. If I feel that someone is exercising away too many calories I let them know the problems associated with it. A few times I've questioned if people may be suffering from exercise addiction. The problem is that exercise addiction is technically anything more than 6 hours per week, which isn't that much!
The problem usually happens when people try to lose weight intuitively without researching the medical issues of different forms of weight loss. However, this can apply to 'helpful advice' as well. Lots of people swear that various diet programs won't work, from low-carb to meal-replacement to calorie counting, when really they're just saying that those programs don't suit them and they don't think they should
work. I was very anti-Atkins until I read more about it. I didn't think Lite n' Easy would work in the long term until a friend of mine lost weight and kept it off following that program.
I think if you voice your concerns in an empathic rather than didactic way people are more open to discussion. Tell them your post comes from a place of caring and concern, or ask them if they've read a certain article that suggests an alternative viewpoint to what they are doing. Hopefully by engaging in conversation you can let them know that they are other, healthier, ways to lose weight.
(BTW, if I'm one of the people you feel is giving poor advice feel free to let me know! I'm open to learning new things about weight loss and I'm always interested in reading new articles