Yes, I think you're right-- MT might be a better use of that time. But then, how many other opportunities look very similar to MT but turn out to be scams? What damage can be done by taking out a payday loan?

I think another way to put what I wrote is that opportunity costs are a luxury one can afford once one has saved enough to move on from focusing solely on catastrophic costs like losing food, shelter, electricity, etc.

OP's friend probably started searching for deals because it returned predictable savings and-- most importantly-- carried very little risk. The fact that she's filling lots of idle time with the same behavior is evidence of her poverty as she is probably not able to take on any greater risk for fear of catastrophic failure.

Thus it is suspicious when the OP analyzes only in terms of opportunity costs. It would be more persuasive to convey ideas in terms of catastrophic risk. But that's a more difficult problem to address and an area where the OP-- like most people-- has little expertise.