You should phrase it in the most kind, politest, non-confrontational way possible. DO NOT tell her she is wearing too much, DO NOT ask her to stop wearing it. Bring it up and ask her if she thinks there is any solution she can think of. She will likely volunteer to wear less or stop wearing it during work hours. Approach her as though you are asking her for her advice on the matter, not as though you are telling her what to do.