What can I do to help my dad?

My mom was murdered a year ago and it hit both of us really hard. Our family used to be really peaceful and happy, but he's totally changed into a person I'm sickened to call my father and I really want him back. What's happening leads me to believe he's partaking in illegal activities. He's a lawyer but almost never see's any clients, and usually the phone was ringing 24/7 but now it hardly rings. Yet he brings home five to ten thousand dollars in one day which he never used to make. He also keeps this money locked away in a safe. Men who i've never seen come over and play "poker" and although he treats me wonderfully and taught me morals before my mom died, he's neglecting what he's been advocating for so long and brings home gorgeous women every weekend and called one a slut in front of me. He started smoking, he started drinking heavily, and he almost never smiles. He's also really mean to anyone besides me. I appreciate him being nice to me but it feels almost fake compared to how he treated me before. Like he was always on top of dates, and would do stuff like tell me "your birthday is in just 6 more days, my little girl is turning 17!" but a week ago he forgot my birthday and I didn't see him all day and it just isn't like him. I love him so much and I just want the real him back. I'm also worried that whatever illegal activity he may be doing to get this huge amount of money may get him hurt. These men he calls his friends who I never met before are very intimidating. I'll be going to university next year and I want to make sure he's mentally secure as I imagine it will be very lonely without any family members.