Just ignore it.
We are biologically unconsciously programmed to assess the opposite sex in terms of procreation to further our species.
That means that, without thinking, or doing so consciously and knowingly, we check out members of the opposite sex for suitability if potential mates.
Women will check a man's height and muscle tone, and genitals for potential protection, strength, and as a provider of babies. Men will check women's breaststroke and hips as potential carriers of babies.
This is normal, biological uncontrolled instinct which we have carried for millions of years.
Adults learn to do this discreetly, adolescents a little less so, due to a combination of hormones, curiosity, and as yet undeveloped discretion.
So your friend is doing what young women do naturally, maybe with additional curiosity after her unintended glance at your 'potential ', so to speak.
You should not take this as a signal of interest, her behaviour indicates that she does not see you as boyfriend material.
A golden lesson for young men is this - if a young woman is interested in you, she will let you know, by signals which you will understand, and covert glances is not one of them.
Enjoy your friendship.