So you say that your gut is telling you something is off, but what about her gut? Its going to be her horse after all. The third or so horse I went out to go look at was an OTTB, and I immediately knew that he was the one I wanted. Everyone else told me I shouldn't, but buying him was probably the best decision I could have made. He's certainly taught me more than I've taught him. I'm guessing this horse has never seen the track? Perhaps she had the professional rider/is selling him because she has back problems and maybe can't hoist herself up....the woman I bought my guy from knew nothing of his riding capability because she had a severe foot injury that put her in a cast for six months, and that's why she was selling him; he was just sitting in a pasture.
Most OTTB's will require a lot of patience and they will lean on you to tell them what they're doing is okay---its kind of the nature of a thoroughbred to want to do everything right. Since he's only five, I'm going to assume he will also need a LOT more hours under saddle before he starts being trained in whatever discipline she wants him to be trained in. If she has the time and the patience, I say go for it. Just get the written contract, and she's all set. The worst that could happen is he's a complete ditz and she decides to return him.