think my dad is cheating on my mom- strategy?

Long story, but I need some advice to know if my strategy is out of line... So, I'm 34 weeks pregnant and my parents just got a place nearby to help us watch the baby for the first year. They also have a summer home in New Hampshire and a little winter home in Florida. Last summer they got a "tenant" living in their garage apartment in NH- she's a masseuse/ yoga instructor who is my age and has a little girl. My dad is apparently super attached to the little girl and even before they found out we were expecting the joke was that my dad was practicing playing grandpa.  
My dad goes up to New Hampshire without my mom from time to time, especially for a little while in the winter because he likes the cold and she doesn't, and while he's there he's mentioned doing dinner and stuff with the tenant and the kid. They've also gone to visit my parents in Florida, so my mom definitely knows her- my mom has actually texted me picture of my dad and the kid saying how cute they are together.  
Anyway, my dad is heading to New Hampshire without my mom this weekend and for some reason it struck me weirder than usual- I've always thought it was odd but maybe my hormones made me extra suspicious? I stalked the girl out on Facebook and she's only friends with my dad, not my mom. He also heart- liked one of her pics (not just he standard thumbs up)  and commented "wow!!" on a cute selfie she posted so I'm totally suspicious now. 
I want to say something but don't want to make it weird, especially because my mom is a strong woman who can take care of herself emotionally and financially, so if something's up I guess she must be ok enough with it (without going into detail I found out back in high school that my parents went through a bit of an "experimental" phase)? But I can't not say anything, so my plan is to approach it I think like "sooo how do we feel about this girl? Do we trust her, or do we think this is some lifetime movie where she's gonna try and steal dad away and then black widow him for his money?" Is that a light hearten enough way of gauging her reaction without suggesting my dad did anything he shouldn't? I'm super close with my parents and so excited for them to move close by to help me raise our daughter, so this has me totally thrown off...