While it is not illegal to pay for someone else's policy, you might run into issues if she's on your policy as well. Car insurance is regulated at the state level and priced at the zip code level. So, if she's on your car insurance policy but not living at your residence, you will probably run into some problems. However, if she had her own policy and you're simply paying for it, there shouldn't be a problem. At the end of the day, the insurance policy really just needs to be paid for. Best of luck!