YANBU. Of course the BMW salesperson is going to convince him that he can afford the finance deal - that's his job. Your job is to actually spell out what that finance deal means to you and your family's joint finances and lifestyle. Has he actually got a total repayment figure. I'm guessing it will be many £k over the £30k price ticket.
BMW 1 series are really overpriced for what they are and not very good to drive. Sounds like a classic case of image concern to me, that he has to have a BMW. Sad thing is that everyone will just look at it and think 'poor man couldn't afford a real BMW' which is insane given that he'd be practically spending 3-series money on it. If he is that set on getting a BMW 1-series, then could he not look at getting an ex-demonstrator or a 1 year old model. It would still have loads of warranty and be just as economical to run, only much cheaper.