Is there ever a good time to tell a woman to stop trying to compete with her daughter?

Say for instance you know a woman who had her daughter at a young age, and there is only 15ish years between her and her daughter. While her daughter is in her late teens the mother is on the shy side of her 30s. The daughter is stunning but the mother is OK. Whenever they go out, the mother tried to wear clingy, form-fitting, dresses like her daughter, and short skirts and booty shorts. Whenever, some guy reacts, whistles, hollers, etc. the mother waves and or blows them a kiss as if she knows it was for her and not her daughter; even when some reference is made to hair color, color of clothes, or eyes which distinguishes them apart. If it appears on the outside to those observing that she is trying to look as hot, or hotter, than her daughter when they go out How or when is it a good time to mention to her it isn’t working like she thinks? How do you tactfully tell her next to most other 30something women she would be a head turner, but next to her daughter she is not ugly but far from stunning according to men in the area you have spoken to?