If they did release her from any book obligations would she owe them any money?
I've no idea how these things work.
Yes, an advance is an advance on royalties. A publisher takes a gamble on a book. Contracts vary widely. Here's a simplified example: a publisher might think we'll shift 100 copies of this book and we'll sell it at £10, and the author gets 10% royalties (£1), so we can pay an advance of 100 copies x £1 royalty = £100
If they only sell 50 copies, the author won't have to pay anything back. The publisher made a bad deal for themselves.
If they sell more than 100 copies, the author will receive £100 as they write the book (that's the advance) then nothing until they sold the 101st copy of the book. They will then receive £1 per copy for every book sold, usually in quarterly or bi-annually instalments.
Some authors also manage to negotiate a better percentage and forgo the advance.
Then there are deals about multiple books, so an author might get a £30,000 advance for three books. Whether a publisher makes the author pay back an advance in that situation really depends on the contract and how many copies were shifted. Let's say the first book commissioned as part of this deal does incredibly well and the publisher makes a load of money on it. If the author then doesn't deliver the remaining two books, the publisher may think that they'd only hoped for £100,000 in sales on book one but ended up making £500,000 so they may cut their losses. But yes, legally and advance is linked to a) a book being written to a specified deadline and b) a calculation by the publisher on how popular they think the book will be.
I'm certain at this stage her advances will be very small fry.
ETA: others have already explained this. I'm slow.