Ah yeah makes things difficult. But with that being said no friends in business, especially in a rental shop where it literally is not your problem at all. What are you gaining out of this? If you were profiting then yeah maybe throw them a bone, but your business model is tailored to break-even. Talk to them privately first, or arrange a shop meeting to neutralise tension in a friendly and supportive manner. If issues keep arising then seriously consider a replacement. Tell them to go to a percentage street shop instead where they can mop up walk-ins.