Can someone please explain this?

Ms. Lemon says that vendors remember if you pay with cash because they assume your checks are no good. As bookkeeper of a reputable business, she needs to pay her accounts with checks. It establishes good credit with the bank and vendors, and makes people, in general, more willing to do business with you. If Mr. Poirot's account was suddenly paid in cash after being paid by check for years, the vendors would think his business was failing, that his credit was no good, and make him pay for everything in cash up front from then on. Word might also get out to other vendors, which could damage Poirot's financial reputation. That is why Ms. Lemon got so upset. She was going to have to call all the vendors Hastings had paid in cash and explain he did that (because he didn't know any better) and try to reestablish Poirot's good credit.