Your primary prompt is configured via the PS1 environment variable. You could insert this in his ~/.bashrc file
export PS1='[\u:\W]\$ '
which will result in something like this:
[username:directory]$ yada yada
then you can add this to his ~/.bash_profile:
[[ -f ~/.bashrc ]] && . ~/.bashrc
When this user logs in, or when you su - to him, he'll have the same prompt.
More about the special characters to configure your prompt