It depends on your cost of capital. On the face of it, if your cost of capital was 7.5 percent x 12 months = 90 percent, this would be true.
This is why people offer discounts for prompt payment.
However, if your cost of capital is 90 percent, you probably have other problems beside late payment.
I fear your MBA friend has mixed up annual and monthly interest rates. At 7.5 percent per year, an extra sixty days will cost you 7.5 percent/12 months * 2 months * 1 euro = 1.25 cents (a bit more, because I haven't taken account of the compounding of interest).
If you just can't get credit or investment for some reason, then obviously the extra thirty days would cripple you. Then it's not a matter of a particular amount of money, the problem is that you just can't trade.