Is it fair for a roommate who's never home to pay an equal share of the utilities?

I have a roommate who is simply never home. She stays at her boyfriend’s house every night, and has only slept here once in 5 months. She sort of uses it as storage. Up till now, she’s been paying 1/3 of the utilities. Is it fair to expect that she keep doing so?

We have a pool, and the pump has to run a lot, which makes the bill higher than average. So she usually has to pay around 100/month.