Macro lens for D5100

First of all, you really do need at least a 100-200mm lens to do good macro shots.  You just won't get the magnification with anything shorter.  The 18-55 kit lens is worthless for macro, and I have a Raynox DCR-250 macro lens (the most magnification) that I can put on it.

I really don't think you need to go to the expense of a detachable macro lens.  A good screw on macro lens will do just as good, IMO, and is much more convenient and light weight -- not to mention much less expensive.  Besides the Canon 500D, Raynox also makes good macro lenses for about the same $70-75 price range.  If you go Raynox, they make an spring-loaded plastic adaptor that you first screw the lens into -- which will then clamp onto a variety of lens ends, from 52-67mm.  If your friend ever changes cameras or lenses, the Raynox will likely fit them OK.  If you choose Raynox, I would, however, suggest you get the DCR-150 and not the 250 lens.  The 250 is an 8 diopter -- which provides huge magnification -- but the DOF is much too narrow.  It is almost impossible to get anything other than parts of flowers or insects focussed well.  I wish I had known that before buying the 250!