I think it's hard to say without knowing what she's getting, when she's getting it and how they intend to process the deduction from her wages.
If they take the whole six weeks and OP doesn't get her statutory holiday then the employer is in breach of contract and has unlawful deduction of wages.
To be honest op, the whole thing sounds a mess and it doesn't sound like you've handled it brilliantly either. I think you need to get some specific, individual advice with all the facts available. You need your contract, any other letters pertaining to your contractual arrangements, any correspondence about your holiday and a clear time line of events.
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I did do the maths, just for anyone interested... and might be useful for op when working out the deduction and how it has been applied.
Presuming 39 weeks a year are worked for term time only with only statutory holiday:
You are employed for your working time (39 weeks)
holiday time (10.77% of working time = 4.2003 weeks)
= 43.2 weeks per annum.
This is then paid across the whole year monthly in equal instalments.
This works out at £231.75 per month (if 12 instalments) of which notionally about 90% is wage and 10% holiday pay (approx £206.79 and £24.96 respectively).