1. Is the agreement of seven months ago in writing? If not then it is the father's word against the mother's - useless.
2. If it is in writing then the Court will weigh that heavily in favor of the father. But hopefully it is not just e-mails or something that are very easy to alter/forge!
3. She is entitled to file for sole custody - so is he. NEITHER parent will get 100% physical custody of their child unless the other has done something heinous. Even people in prison can be allowed supervised (obviously) visitation with their own children.
4. Best interest of the child is usually the status quo as set at the original hearing. All you need to do is show a change in circumstances - for example, father used to live hours away and now lives within 20 miles.
5. It is not so much you have to prove that you are the better parent - more that you have to show the other parent has demonstrated something that should limit their time with the child. BUT it has to be something VALID - not emotional but fact.