Your friend is in quite the pickle.
As they say: good, fast, cheap. Pick two.
She wants the 600mm equivalent reach, and she wants good image quality. *And* she wants something light.
I'd say her best bet is probably a 1DIV and a 400 f/2.8 II.
But it won't be cheap.
If she wants to do it on the cheap, she's either going to take a hit on the image quality or the weight.
Once you stick a supertelephoto on a monopod, weight isn't as much of a factor -- and, really, who even wants to handhold a 300 f/2.8?
So, with that in mind...well, KEH has a 600 f/4 right now for $8K, or a 400 f/2.8 II (non-IS) for $4750. I'd recommend the 400 with the 5DIII and a 1.4x TC, but only because I think she'd find it more versatile. But if that TC would remain permanently mounted, then either swap the 400 out for the 600 (best IQ and AF performance) or the 5DIII for a 7D.
Honestly, it doesn't sound like she'l be happy with a 300, no matter what. And, continuing in the honesty vein...if she's considering a supertelephoto but the price and / or weight of the one versus the other is a consideration, then she's not going to be happy with *any* supertelephoto, *including* the 300.
One last option I'll throw out that might actually be what she's looking for, though she probably doesn't realize it: the 7D plus the 400 f/5.6. Yes, it's optically slow, but it's got lightning-fast AF (or so I've been told) and it's easy to handhold (at the kinds of shutter speeds she'd need for the action). Damned good IQ, too. This is, of course, assuming that she only shoots in good light...but I'm guessing that polo is going to mostly be outdoors in good weather.