
Expose your pictures properly and this will be much less of an issue.
I think the issue here is that she is ALREADY doing all she can to "expose properly" f/5.6 is wide open for that lens when it is on the high side of 100mm She is shooting at 1/50th of a second, which is pretty slow for over 100mm - already twice as long as the reciprocal of the focal length, and that doesn't even count crop factor! And, she is already at 640 iso, which is getting very close the end of the useful range on the 40d.
So, tell me, which of these factors should she adjust to "expose properly".
Rad