A) When criticizing somebody even constructively or for benefiting them make sure it is worth it. By the fact you have to ask this question here in the coffee room shows that you might have doubts about what you are doing or thinking of doing. Speak to an objective outsider without naming names and who will not realize who your friend is and describe the problem.
B)You have not stated if it is a flaw such as weight or manner of dress or a character flaw. Regardless of issue approach it gently as perhaps your friend is already cognizant of the issue and is not able to help themselves but wants to change,or just has not reached out yet or does not want to reach out to friends for help. Whatever you do speak in a manner non-embarrassing to your friend and in a manner that you do not come across as holier then thou and a know it all (A while back I had a friend who one of our chevra felt had to be spoken to about his short temper, unbeknown to us this friend of ours was well aware of his tendency to blow up and had actually begun speaking to a therapist about how to help him control himself.He obviously never discussed this with us prior to one of us speaking to him, as he felt that it was not for public knowledge.)
C) If you do bring the issue up with her even if she expresses her desire for your input don’t constantly bring up the issue with her afterward.If she does not want your help but does not totally blow you off perhaps she herself will bring it up to you on a later date after thinking about it for awhile or if after awhile you still see your friend not changing and it will not make the friend totally embarrassed you might want to bring it up again of course you will have to be the judge if it is worth it or not
d) Make sure you have a way to help or guide her in a soft manner if it is an immediate issue that can be dealt with. A terrible thing would be to point out a flaw and then not know how to handle it, it will only further complicate the fact that she is aware that she has this flaw and my friend is pointing it out but I still have no ideas how to change. Even if you yourself cannot help, think of a person she could speak to or a book she could read.
e)All suggestions should be made in a non-forceful gentle manner as while you might think they are great and correct (WHICH THEY ACTUALLY MIGHT BE!)she might not. If she feels your idea is ludicrous then she might dismiss the meaningfulness of you wanting to help her as well
f)HATZLACHA! It is a tremendous Zechus to help a fellow Jew