Pretty much all proper macro lenses deliver very good image quality. That's one dimension your cousin doesn't have to worry about.
Most "proper" macro lenses will give you 1:1 magnification. That means that, when the lens is focused at its minimum focus distance (MFD), you can fill the frame with a subject which is the same size as the camera's sensor - in the case of your cousin's 50D that's about 22.3 x 14.9 mm. Of course you can achieve a lower magnification, if you want, by not focusing in so close. But don't get fooled by consumer zoom lenses that have the word 'Macro' painted on them. They'll tend to deliver slightly higher magnification than 'ordinary' lenses, but maybe only 1:3 or so.
The main difference between macro lenses is that, while they will all give you 1:1 magnification, the MFD at which they do that varies with the focal length. In very round numbers, a 60mm macro would have a MFD of about 20cm, a 100mm macro would have a MFD of about 30cm, and a 150mm macro would have a MFD of about 45cm. But bear in mind that that's measured from the sensor, not the front of the lens. The distance from the front of the lens to the subject at MFD is called the working distance, and it's going to be around 10 cm / 15cm / 25cm for 60mm / 100mm / 150mm macro lenses. The working distance can be very important when it comes to getting light onto your subject, and also if you want to avoid scaring off skittish insects.
So which to buy? Personally I think "short" macro lenses, whilst cheap, can be too difficult to use. I think something in the 100mm range is very usable and you're spoilt for choice: the Tamron 90mm, Canon 100mm (two versions) and Sigma 105mm are all very good. I think the Canon 100mm "L" series is probably the best, but the Sigma probably offers the best value for money. But you can't go too far wrong with any of these.
The 100mm 2.8 non-L is great value used. Afaik the main difference between that and the L version is OIS, and built quality. I don't think there's anything significant in it image quality wise. OIS can be nice for framing close up images, it tends to help you steady yourself let alone the actual frame. But it is over double the price. The 60mm 2.8 also looks pretty good, sure you have shorter working distance but I've often shot bugs in close enough the front element [reversed lens, macro rings etc] was almost touching them. If they don't 'bugger' off by the time you get within a couple inches, they tend to stay even when you get closer. I think they start admiring themselves in the glass Blocking light is the bigger issue, but if you are using off cam flash, which you really should for best macro results, then that's not such a big deal either.
I own a canon 7Dii and Sony NEX5R and I use my canon ef 100mm f2.8 macro lens on both. A pal of mine has the 100mm f2.8 L IS USM.
The L version is a great lens and the main differences I can tell from using both is the L has better built quality, image stabilised. This has greatly when hand held over using tripod or mono pod.
Bang for buck the ef 100mm f2.8 ( non L ) is very good value for money. Now when you throw in the sigma 105 that is also an extremely good lens !
As mentioned above you can not go wrong with 100mm, 100mm L or sigma 105