How can I help my mother escape my dad's abusive behavior?

OK, so I have two of the best parents in the world-my mom's always been stern, but wonderful and I've always been a Daddy's Girl. I've considered my dad a greater man than my husband and love almost everything about him until my mom started letting me in on his behavior that's steadily grown verbally & emotionally abusive over the past 1 1/2 yrs. My dad got a good job as a superintendent for a well-to-do construction co. complete with a shiny laptop, nice paycheck, etc. And she says he's become mean and cold to her, especially this year. She stumbled onto on his laptop, (something my father NEVER WOULD HAVE DONE), he has become very secretive, WILL NOT open his wallet without turning his back from her, yells and screams at her when she simply asks him if there's another woman, and she has seen miscellaneous odd things he has looked up online in his history such as apartments in the city. He has most recently "Forbidden" her from even asking when he will get home everyday, she's not to touch his cell phones, computer, or get in his vehicle. She has found paycheck stubs of his, proving he makes PLENTY especially in situations like this past xmas when he claimed he had no money for any presents-my mom found a stub that shows he received a $1000 bonus at the beginning of Dec. He says his money is his. That he deserves it. I have found that even if my mother wants a bottle of tylenol, he makes her pay him back for it!!!!! This is not my father!!!!!!! It is like someone stole my dad and replaced him with an jack. What broke my heart was when my mom went to the dentist due to some pain in a tooth and swelling, the dr. told mom it would have to be pulled. This was going to be nearly 500 and my dad refused to help her. That is not what marriage is about. My mom is nearly 50 and went from her dad's house to my dad's. She doesn't have $ for a lawyer and she is making herself physically ill from all this stress. I want to know 1.) WHAT IS HE HIDING? 2.) IS HE CHEATING ON HER? (Apparently he is all of a sudden 'different in bed') 3.) DOES SHE HAVE TO PROVIDE PROOF OF ALL OF THIS FOR THE COURTS? (Being that she magically gets a lawyer) Thanks so much