If you are comfortable with public transportation, you can also use the subway system out of south station. The red line connects to south station, which then connects to the green line (either D or E line, which goes to the Longwood area/Harvard med if that's where your student host is). It's pretty easy to figure out and cheaper than a cab.
As a Bostonian, I prefer calling cabs than trying to find one. But south station is definitely a bustling transit spot so you'd probably be able to hail one off the street as well.