Oh, double standards. They even exist in the French language. Il est bon. = He’s a good person. Elle est bonne. = She’s good in bed. So, if you want to say that your female friend is good at doing something, focus on the verb rather than trying to translate word for word:She’s good at dancing. = Elle danse bien.To say that she is good at something (e.g. math),  make sure to elaborate with one of the following expressions:Elle est bonne en (maths).Elle est forte en (maths).

Oh, double standards. They even exist in the French language.

Il est bon. = He’s a good person.
Elle est bonne. = She’s good in bed.

So, if you want to say that your female friend is good at doing something, focus on the verb rather than trying to translate word for word:

She’s good at dancing. = Elle danse bien.

To say that she is good at something (e.g. math),  make sure to elaborate with one of the following expressions:

Elle est bonne en (maths).
Elle est forte en (maths).