It is right that as soon as anyone has purchased a fish and money has changed hands, the new owner is 100% accountable for the animals welfare under the Animal Welfare Act. This is actually why Aquatics and Pet Shops have absolutely no obligation to replace fish that died or take fish back, the way the law is written is that once money has changed hands, responsibility has been transferred. If the fish die, it's actually the new owners responsibility, they are actually legally responsible.
Pointing that out shuts people up when they complain because their fish all died after two weeks, an aquatics shop is just like that, they are there to sell livestock and dry goods, they are not responsible for the care of the fish once they have left and so if someone hasn't bothered cycling their tank properly... not the shops fault. In the same sense that if you buy food from a supermarket and when you take it home and cook it and it doesn't turn out right, you don't just go marching back down the supermarket!
That aside, as much as fish are legally counted under the welfare acts, it just isn't enforced. Its not great at the moment, bad pet shops can be reported! All shops selling livestock have to display their petshop license which has contact details on it for the council and the person who granted the license, if you aren't happy, that's who you call (and they are strict as heck people and will come straight out to check!), the RSPCA would mostly just laugh at anyone who called about a fish...
And yet there is no real way that the shops can report the fish keepers without being laughed off.
Unfortunately for the fish, your job is to care for them at the shop and to sell them to the most responsible home you can, you are well within your right to refuse the sale of a fish to a completely unsuitable tank but be careful with that if you value you job. Once the fish leave your premises, they are no longer your problem to deal with, you learn from it and you move on. You make the situation clear with the other staff (not to sell her fish) and with the customer (that she cant buy any fish until she sorts out her stocking).
The main thing you have learnt from this? Other than some people are just idiots... is that you need to learn more about cichlids. We have a great cichlid section here with some really knowledgeable members, go knock yourself out asking questions!