I know your friend is probably sold on the 2.8 ... but, he/she should really at least think about the 300mm f4 at a fraction of the cost.
You're talking about one stop in speed which could probably be compensated for by a jump in ISO.

I shoot lots of sports and really love my Nikkor 300mm f4.  With that and my 80-200 f2.8, I'm pretty close to satisfied ... Photographers are NEVER satisfied, you know.   

PS:  80-200 is not a mistake.  I went with the older (non-VR) model for less than $1,000.

ED:  I'm not a Sigma fan and mine should not be considered an objective opinion.

It might be cheaper to buy a new camera that can handle high iso.

I would stay away from anything else than f2.8 for shooting sports at night. I don't know about Brazil but here in Europe, more often than not, you wish for f1.2 I can't recommend using a TC on an 70-200 because you lose a stop, obviously and the AF gets slower, not-so-obviously.

I don't know the Sigma 300/2.8 but from first hand experience I can recommend the following:

-) Canon 300/2.8 IS Mark I (the old one): Available only as used, sharp, ultra-fast AF, euro prices around �3.400,-

-) Canon 300/2.8 non-IS (the very old one): At least as sharp and fast as its IS cousin, just without the stabilization. When photographing sports I rarely felt the need for IS, but YMMV. Available only as used, price around � 1.600,-

-) Sigma 120-300/2.8 DG OS: Almost as sharp and fast as the Canons above, with OS and the option to zoom out if the play gets really close to you. Definitely go for the newer lens with OS, because it is a lot better than its non-OS predecessor. Price in Europe is around �2.200,- new.

Thank you all for your replies. Yes, 2.8 is really a must, I guess. When I go to a stadium - on the stands and not on the field, however - the light is a bit low even for a f/1.8 at ISO 1600.

Your comments on the 120-300mm have been quite interesting.

Have you ever used the 120-300 in low light conditions, like in a stadium or maybe indoors (not studio)?

Yes, I did - AF is still fast enough for sports though a little slower than the Canon 300/2.8. One word of warning: I owned too models of this lens: the old non-DG and the current OS version, both required quite heavy microadjustments in the camera for the focus to be spot on - so if your friend's camera doesn't have that capability I would test the lens before buying it.

If you're interested I can e-mail sample photos to you in the evening (my time, twelve hours from now )

I have no idea of what adjustments you are talking about.

In newer Canon cameras, like the 7D, you can microadjust focus and correct for front- or backfocus on a scale of +20 to -20 (arbitary units). So, if it should be necessary, your friend's camera can do it 

Thanks everyone for your help. My friend is getting the 120-300mm. The microadjustment of the focus is a novelty to him as well. Now I wish my 400D had it too!