Strictly answering your listed questions, without comment on what the 80GB worth of files might be in regards to TOS (which is another consideration you may not have considered):
There is always a risk involved when you let someone other than yourself have access to an account for which you are responsible. Only you can decide if the risk involved is acceptable to you.
Create a new sftp/ftp only user and provide the person with that users credentials (and keep them for yourself also). They can then upload the files to a directory owned by that user, and they can access only that directory. You can also access that directory by using those same credentials (as that user).
After the job is over, set the permissions on the files so that you can copy them to your user directory,copy them to your directory, and then either delete the extra user altogether or change the password in the control panel so that the other person can no longer access the directory or the account.
This is answered ias part of the answer to question #2.