Realistically mate, whatever they buy now will be unsuitable in a year's time. IF they keep riding, they'll get fitter, stronger, discover interests that may not appeal now. Few people can accurately predict how they're going to use something new and not a lot of experienced cyclists get it right either.

If you can, get them to buy from a bike shop. Theoretically, the bike will be of decent quality, will have been built properly and they can take it back for maintenance ... but you know all that. After that, just get them to look at a lot of bikes, the more the better, and then buy the one they fall in love with. Bearing in mind my warning in the first paragraph, bottom of the series or one step up is better than buying the top line model, purely because they will probably want to buy something else next year, but hey, it's their money, let them spend it how they like.

The actual model, brand and style isn't that important. What's important is that they love their new bike and just can't wait to get on it and wear it out. The emotional connection is more important than the physical. There aren't many truly bad bikes out there, particularly in bike shops. Make sure they test ride it (on the road, not on a trainer, it makes a big difference) and make sure they are comfortable. Let the adventure grow from there.

Richard