"With someone's money" means would be he is borrowing someone's money by saying that he is going to use it on something that is really important causing for the his friend or love ones to lend him some, but eventually he just use it on gambling and mournfully he lost it all. So, the result would be obvious, the person who lends him some money would be mad or disappointed because he lied just to have something to gamble.

And that is the worst thing that you could do to someone you know, which is to lie. The worst case scenario that could happen is that, that person would be mad at you forever, or even if he forgives you, he will not forget what you did and not trust you again just like what the other had already mentioned.