I would just say "Man I am so sorry you got such a rotten cake from her....btw here's my business card" hahaha
fairmaiden-you are too funny! I would give out cards, but unfortunately I'm not legal yet-I'm constantly turning down business which makes me wonder if I should just start accepting, but I'll be legal soon so I've just been trying to be patient and hoping all the people that want my cakes now will come to me again when I'm licensed. 
My hubby and I have our own small, in-home business (not cake-related). One thing we have been told from Day 1 is that there are TONS and TONS of customers out there for business. But a business can't be built and sustained on just customers. One needs good, reliable clients that will continue to come back time and time again to sustain the business.
If her cakes are as bad as you feel they are, time will eventually prove that. Customers may never go back to her, nor will she have repeat clients. Over time, her customer base will dwindle and nature will take its course.
Your goal is to try to turn her customers into your clients. Good luck!!
There's a storefront bakery in my city that makes cakes that look like cow patties. And people still buy from them. To me, if they have the money to open a bakery they should just hire a decorator if they don't have the skills themselves...but who knows, maybe they like the way their cakes look.
Which bakery is that. This I gotta see...LOL! PM me with that info if you don't want to post it publically.
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