Maybe, but there are lots of ways to calculate opportunity costs.

The OP said his friend was making more than $150k/yr. So if he estimated his potential net worth in thirty years staying course versus his estimated net worth in thirty years with an MBA, then discounted it to present value, a difference of $500k is somewhat reasonable.

There are plenty of other, equally valid ways for a person to calculate opportunity costs. It depends on what that person values. For example, lost wages that would have paid for vacations would not necessarily show up in a comparison of net worth, but it is certainly an opportunity cost for some people. Or having to put off starting a family, or many other milestones in a person's life.