A girl's hymen is in their vagina. The hymen is what makes a virgin
girl tighter than a girl who has already had sex, and once a girl has
sex for the first time her hymen breaks and she no longer has it. (This
is also why a girl tends to bleed after her first time.)
So yes if she
has only had anal sex and her hymen in her vagina has not broken yet
then she is a virgin.