My condolences for your loss.
If you file to open an estate in probate and get an appointment as estate representative then you will have legal authority to marshal his assets, including his papers and accounts. If the girlfriend refuses to cooperate the court can hold her in contempt and sanction her with fines or even jail.
You will need a probate attorney in Florida, as pro se probate is not permitted, under most circumstances.
If you need assistance finding local counsel try Martindale Hubble (site rules prohibit us from referring specific attorneys). Many attorneys themselves use this site to locate attorneys outside their jurisdiction or expertise:
It is a huge worldwide database searchable by location and specialty. The attorneys are peer rated. So, they represent the top of the profession.
The Florida Bar maintains an excellent online referral program:
FLORIDA BAR ATTORNEY REFERRAL
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