Two things. Just because an MTA is included in the default install does not mean there is anything special about it. With sendmail it goes more to tradition than anything else. Historically, sendmail has been included in Unix for decades and is expected to be there.
That being said, You may or may not know that OpenBSD is an operating system. Linux us not. It is a kernel. As such, before any port is added to the default install or as an option in OpenBSD it is audited by the OBSD team to be bullet proof. This is true of all OBSD software and this is why it is one of the most if not THE most secure OS on the planet with only two remote holes in over ten years. That does NOT mean it is any easier for a n00b or even a seasoned professional to configure.
We can certainly let him make his own choice. There is Exim and several others as an option, however I think you may agree Postfix has a great user community and is very well supported with tons of tutorials on the web. If he should choose Postfix, I'm ready to help.