It depends on his macro lens. If he is using short to medium range lenses, 50mm - 105mm, extension tubes will be useful. If his lens is between 180 - 300mm, he needs to extend it by 62mm to get 1:1 which means that the aperture is badly affected by a factor of 6.
A better option is to get an achromatic lens attachment like the Canon 250D or 500D. They are basically close up filters with two elements. 250D (+4) for 30-135mm and 500D (+2) for lenses above that range. The lens' aperture will also not be affected.
For the Sigma 70 - 300mm f/4.5-5.6 Macro APO Super II, which can do macros up to 1:2, adding a Canon 500D should bring it up to 1:1.
close up filter is just a piece of magnifying glass. +1, +2, +3, +4, +5, +10 (or so called Macro Lens attachment, ability to achieve up to 1:2 Macro on 50mm lens) diopter means the amount of magnification you can get, of course there will be slight degrading of quality. However, if you get a 2 elements close up filter (Archomatic), the quality loss will be alot lesser.
ok thanks for your information. I have a Tamron 70 - 300mm which gives 1:2 at 300mm so what advice will you give? deadpixel i dont quite understand what you are trying to say about the aperture being affected by a factor of 6.