If your friend has blue eyes, it means that her genotype for eye colour is homozygous recessive (bb). This is because brown eyes are dominant over blue eyes, so in order for someone to have blue eyes, they must have two copies of the recessive allele (b).

If your friend's parents both have brown eyes, it means that they are both heterozygous for eye colour (Bb). This is because they must each carry one copy of the recessive allele (b) in order to have passed it on to their daughter.

When two heterozygous parents (Bb) have children, the possible genotypes for eye colour are:

* BB (brown eyes)

* Bb (brown eyes)

* bb (blue eyes)

As you can see, there is a 25% chance of having a child with blue eyes when both parents have brown eyes. This is because the only way for a child to have blue eyes is if they inherit two copies of the recessive allele (b), one from each parent.