Thank the shadchan for the suggestion & tell her how much you appreciate the time & effort she put into suggesting such a quality suggestion for you. You can say that you did quite a bit of reseach into the girl & really heard very nice things, but based on the info you got – it sounds like “hashkafah-wise” it isn’t on target. You don’t have to elaborate, just firmly tell the shadchan that this shidduch isn’t going to work & you want to save everyone (you, the girl & the shadchan) the time & headache of going out with a person who isn’t on target for you. You should thank her again & say that you look foward to hearing other suggestions from her.
