If she’s never taken it before it’s generally recommended for women to start at 5mg/day and then bump up to 10 if we tolerate it well. 10mg is a typical dose for women, but she may get some sides and she might not like some of them, so best to start a bit lower.
There are several threads on var in the Fem Chem forum if she wants to read about what she can expect, but common sides include clitoral enlargement (this goes back to normal after you stop taking it) and interrupted or less heavy menstrual cycles.