I do work with social services and you're talking out of your 'arris love. It's very difficult to prove sexual abuse either way, male of female, it's quite rare there are physical signs of abuse. In fact, a lot of the physical signs of abuse can also be physical signs of some other medical condition too, you have to interpret them in context of many, many things, not just the allegation. It's not easy to prove sexual abuse period, it makes no difference whether the alleged perpetrator is male or female.
The literature tells us that female abusers and female paedophiles (not always the same thing, same for male abusers and male paedophiles) are quite rare. I suspect that further investigation is warrented as I do wonder if there are more cases out there as it's still a taboo subject but CAIT are very unlikely to drop a case depending on the sex of the perpetrator, they'll drop a case because of lack of evidence or disclosure, but not because the perpetrator is male or female.
I imagine more work and research is being done on the subject of female abusers but your assertations of 'my mate says' are pretty rubbish I'm afraid.