|Login as administrator (you will need to know the password - it may be blank) and set up new accounts in the same names. |
If you are lucky he may not have deleted the user files.
If he has NOT deleted the files then you may need to take ownership of the files.
If he HAS deleted the files then before you do anything on that system you might want to run some recovery software on that PC to try and recover the files if they are important. In the meantime do NOT write anything to that hard drive / partition where the files were resident until you have recovered your files.
☺ When everything else fails, read the instructions.