The amount being sued for is nuts. But recognize that that's just an opening figure. When this matter is settled or a verdict comes in, it'll be for a lot less than $400k.
As for a lack of a written contract, it kinda doesn't matter, and it kinda does, see:
http://www.law.cornell.edu/ucc/2/article2.htm#s2-201
I have a lot of trouble believing that there's absolutely nada in writing. There may be a signed receipt or a cancelled check, or evidence of a wire transfer. Or a bill of sale -- that'll probably do. The court is probably not going to limit damages to $5k, particularly because an equitable remedy (such as an injunction or a requirement of specific performance) won't fly as the wedding is over and cannot be duplicated (don't tell me about renewing vows; the court will not see it that way).
Plus, the UCC (Uniform Commercial Code) applies here because it applies in 49 states and DC and big chunks of it apply in Louisiana, but the services portion of the business arrangement does not come under the UCC. Services include things like delivering the flowers, arranging them, that sort of thing.
I still say it's judgment for plaintiff, although she'll have more trouble proving her claims than I'd originally believed. Those problems with proof are her own damned fault.