It sounds like a general admin fee rather than just for credit checks. Some charge one fee for everything and one place I used charged an admin fee then a separate fee for each credit check.
If she doesn't have a credit history or has a bad one or hasn't been in a job for more than a certain time, often 3 or 6 months then she may need a guarantor.
Credit history is a funny thing. When my ex and I went to ask about possible bank loans as a means of financing a car puchase at our respective banks, I, having no credit cards, loans, bad history etc... and on a far higher salary didn't get anywhere near as good a deal as my ex who livces in his overdraft and has credit card debts as he had a credit history and I didn't!
When I had a flat I was about 27, a qualified solicitor, no credit cards, loans etc... but as I'd just started a new job and hadn't been there 3 months I had to have a guarantor.
I'm flat hunting at the moment and it's rare to find agents who don't charge a fee nowadays. I've found ones lately ranging from �60 to �150, often with VAT on top.