I took 3.5 years of Spanish in high school. That was 20-odd years ago and the only 3 languages offered were Spanish, French, and German; and none of the others interested me. By the time I was a senior, I was relatively fluent as long as people spoke slowly at me, and were willing to put up with what I am sure was a pretty awful accent. My grandpa volunteered at a church-based community service center/relief agency and would often take me along during "migrant season" (we have a large cherry-picking season mid-summer that attracts farm workers) to help translate for the Hispanic farm workers.
That said, I have used it very little since my high school days, and have lost almost all of it. I can still translate a little, especially if it is written, or spoken very slowly. I can make myself understood enough to request a bathroom or a policeman, or to order food. I can still count and identify colors and say basic things like good/bad, hello/good-bye, slow/fast, left/right, thank you/you're welcome, etc. That's about it.
DD18 is taking French and is on her 2nd year. Her sole reason for taking it is that she wants to be able to travel in Europe/France. Honestly, I think she has learned very little.
I would let the student choose, but know that Chinese is HARD, and if she doesn't have a natural aptitude for languages it might be discouraging. Also, a lot depends on what she thinks she may do as an adult. Where will she live? What career will she have? In the United States, in a lot of areas, Spanish will be a more "useful" language. In the business/International community, Chinese might be more widely used.