I took both Mandarin and Russian to Masters level, but not in the UK, across four different countries, and took the two languages about a decade apart.
I also taught Mandarin in uni in the UK for a couple of years, but this was in the 1990s, so much has probably changed, including teaching and learning resources, as well as job opportunities after graduating.
All those caveats aside, I do have experience in learning both, and have a decent idea of what some people ended up doing afterwards.
First questions:
What else would he be studying?
Would it leave sufficient time for language learning? Chinese in particular needs a heck of a lot of extra time put in by the student just for sheer memorization of characters and vocab. Of the top of my head, I would say at least two extra hours put in by the student per hour of taught lesson. Russian also needs time, but not quite as much.
Will whatever else he studies lead to a job where the language could be incorporated? E.g. geology, some type of engineering, law, business, computer science?
Can he make himself passionate about a subject to the extent that he would go above and beyond to learn it? I.e. spare time spent watching movies in the language, spending one's remaining spare time hanging out with native speakers of the language? (If the answer is no, then he probably shouldn't take Mandarin).
If you do only a BA you will not come out of it with anything like the type of fluency you would get in an easier European language. You would still be deciphering texts fairly agonizingly. Spending a year in a relevant country and studying the language full time there would make a very large difference to that, though, as long as one would spend that time studying intensively and not fooling around (as many do).
Will post more in a moment, as want to read what others have posted meanwhile.