When you're in a hole, dig faster?
Better analogy: when you're in a hole, get some explosives. They might blow you to pieces, but maybe real world physics = FPS engine physis and you can rocket jump out of that hole
seriously though, I don't see a difference in principle between not investing more than you can afford and not taking a loan higher than you can afford. If he knows he can repay the loan within tolerable living conditions in case his investment fails, it's not necessarily an irrational choice.
I could afford the loan out of my current monthly salary if I gave up alcohol and went onto a brown rice diet for a year. Plenty of options and not all of them include halitosis