Invoice factoring?

My friend got in a bit of a bind at work, and was telling me about it. Apparently someone hasn't paid up (She works in the accounts receivables office). So now she's faced with either having to use the receivables as collateral for a loan, invoice factoring out to some kind of other company that will buy them, or a couple of other methods I didn't understand. Hopefully it all works out. She's been pulling her hair out!