I think my dad is cheating on my mom :(

My dad is now texting and talking on the phone with his ex wife (from years ago!) my parents have been together for over 10 yrs.. his ex wife was married to someone else which passed away about a year ago as soon as this happened all of a sudden she started to talk to my dad about their son (he’s gay and she found out & reached out to my dad) i totally understand they might speak bc of their son but i have been noticing when my dad gets from work he speaks with her for about an hour 🫠 once my mom gets home he hangs up.. the other day his phone was ringing and it said Anna (ex wife) and my mon asked who’s calling and he said “Mario” i clearly seen it said Anna so I pointed it out and he didn’t say anything i find it weird that he hides it in my opinion if they just speak about their son he shouldn’t be hiding anything right ?! A few days passed and he asked if I could help him add his boss new phone number so I did and next thing you know his ex wife texts him “😘😘🥰” which I found even more weird and suspicious why is she sending those emojis !?? I don’t know what to do he has been acting really different with my mom and gets mad for no reason lately I have been really sad bc of other things and now this is making me even worse how can I tell my mom ☹️