Surely the whole point is after she blows it all on sweets and chocolate she realises she has no money left to go out with friends or top up her mobile and is stuck in the house?
We learn best from our mistakes.
Sit her down and work through what you think she needs for a month. Explain to her she is getting enough money for 1 month to cover McD's, cinema, phone, school lunches or whatever else she needs it for and its up to her to manage and make decisions. Give her examples, if she buys sweets everyday at school lunch at just £1 a day not only is it too much sugar and not healthy it is £20 gone for that month which could be a cinema trip, a new tshirt a bit of make up etc.
Tell her you will help her budget for the first couple of months and warn her she will get no further money until the following month if she blows it all at the beginning on crap (if she blows school lunch money she will need to borrow from following months pocket money to eat!). You should also have the passcodes to her bank account(s) to see the transactions/balances.
We did this with ds around his 14th birthday. He has a savings account and a debit card account. It had the opposite effect you are worrying about, he has been mature about it and unexpectedly quite frugal now its "his money". It stopped him buying those sugary sports drinks at school lunchtime too as he realised how much money over a month they cost, he's also much more careful with his mobile phone data allowance as he knows he has to pay for any top ups needed above his contract.
She will make mistakes, but keep an eye on what she is spending it on for the first few months and give praise or guidance/suggestions as needed.
If she would run out and blow the lot on sweets also deal with the healthy eating issue separately.