As has been stated, nude photos of under 18's are legal.
Indecent or sexualised photos of under 18's are not.
The problem is that
indecent is open to interpretation. Now normally, if I am not mistaken, a court would take the interpretation of a majority of right-thinking peers to decide whether it's indecent or not.
Just look at the amount of replies on here and how many of them would consider it dodgy ground? That, I think is your answer, and is likely to be the decision of a court IMHO.
Nevertheless, that's all conjecture and guesswork and really still doesn't give an answer to the problem.
I understand you have this girl's interests at heart when you say you're considering it. However, would you seriously risk a criminal record and a ruined career, just to stop her going to another photographer that might be a bit dodgy? As much as I am kind and helpful to a fault, I know I wouldn't.
And no court in the land would allow that as an excuse should it ever come to light.
The thing is, it's not just a case of you doing it this once and no one knowing. The shots could materialise in 10, 20, 30 years time. Do you really want that hanging over your head?
I advise you to firmly refuse to do any under 18 work, and express to her that not only is it illegal for you to do it, it's illegal for anyone to do it.
I would also explain that her mum's consent means nothing in law, if anything were to happen, her mum could be put on the sex offenders register (a dramatic statement but entirely possible). Maybe not now, maybe not in the near future, but again, 10, 15, 20 years down the line.
Look at how many people who worked with youths in the 70's and 80's are now being arrested and charged with child sexual offences.
OK, taking some risque shots of a 17 year old is clearly different to sexually abusing an 8 yr old. However, the law covers both, and a criminal record can be given for both. The SRO can be given for both offenses too.
Two opposite ends of the spectrum I agree, but both still illegal, and therefore taking the photos IMO is certainly not worth the risk.
On a final note, please don't let this thread descend into a catfight or a sick-joke-athon

It's a good discussion and does serve to highlight the problems that photographers face.
Unfortunately, it's a sad fact of life that we must protect ourselves first and foremost, because it's a really dangerous world out there, where lives can be ruined by even the smallest headline or accusation.