Speak your mind in a concise, professional manner.
Present them with the cons of what they have now, and the pros of redoing it from scratch.
If they have any intellect, they will very quickly realize that molesting a turd into a polished turd is no good
*Edit: they are looking to hire you because they know you have skills, and they know you don't fuck around, and they would be paying you for a premium service, so I don't think you have to worry about demeaning the previous designer if you always start your sentences with "Well, in my professional opinion, this is terrible because... "