sudo -i -u foo_user

foo_user@ubuntu:~$ sudo -i -u admin1
[sudo] password for foo_user:
foo_user is not in the sudoers file.  This incident will be reported.

su admin1
Password:

-i [command]
               The -i (simulate initial login) option runs the shell
               specified by the password database entry of the target user
               as a login shell.

su admin1