Often times people who are doing well will refer to themselves as lucky and it's a nice way to recognize that someone who was equally talanted and worked as hard may not end up with the same rewards.
Well, I think that's exactly the issue. When people attribute their own successes to luck, it's to minimize theit own achievement so they don't seem like they're bragging. But to minimize someone else's achievements for them seems a bit rude and dismissive.
I totally agree.
Some people are thin because of hard work. Some people are thin despite eating twelve deep fried big macs a day. Some people are not thin despite hard work. Some people are not thin because of eating twelve deep fried big macs a day. I don't think it's polite to comment (1) on anybody's size or (2) on what someone has done or not done to be at that size (with the caveat that sometimes there are appropriate situations, like if a good friend takes up running and losing 100 pounds - but as a general rule with acquaintances, just don't comment).
And describing someone else's "success" as lucky is really annoying, whether it's about looks, size, health, happiness, family situation, job, marriage, wealth, whatever. When you say a person is lucky to have X, you're implying that they haven't done anything to deserve or earn X.
Finally, it's not rude to have ANY kind of thought in her head. She can literally be thinking "I'm lucky I'm not a fat ugly loser like you, jerk!!!!" and while I wouldn't describe that as a pleasant or kind reaction, I don't think the act of thinking can ever be rude, no matter what the thoughts are.