why don't they just borrow it from lisa?

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I wondered this, too. I think they could have thrown it in there somewhere, in the midst of all the chaos.

Well then there wouldnt really be much of a script. if they borrow the money from Lisa, they wouldnt have to go out and get those stupid jobs. be escorts e.t.c lol

That makes sense for more at the ending when Zack finally revealed that all the money was gone, they could have just asked Lisa instead of going for the tacky wedding. However, he didn't want Kelly to know about the money and if he asked Lisa, she most definitely would have told. Or it would have been classic SBTB episode where Lisa would have been dying to tell Kelly but couldn't and would have spent the next hour or so trying not to tell her before eventually caving in.

Josh:It looked like you wanted to hug me.
Leo:Oh, man, did you read that wrong.

You're assuming that Lisa has the money. If you've watched the series, you'd know that it's Lisa's parents who provide her with the money, and that new BMW. Lisa spends all of her money when she shops for clothes, make-up, etc.

Lisa would not have had the money to help them out.