Answer
You should tell her to get a 3.9 kg rope.
Work Step by Step
As we know that
$\frac{v_1}{v_2}=\sqrt{\frac{F_1}{F_2}}$
$\implies \frac{v_1}{v_2}=\sqrt{\frac{m_1g}{m_2g}}$
$\implies \frac{v_1}{v_2}=\sqrt{\frac{m_1}{m_2}}$
We plug in the known values to obtain:
$ \frac{18}{30}=\sqrt{\frac{1.4}{m_2}}$
This simplifies to:
$m_2=3.9Kg$
