Do you think I should contront my mom about cheating on my dad?

Well first off ill say that im almost 20 years of age and this morning i was brining my mothers cell phone to her as she asked me too. and i opended it to see if she had ne missed calls. and i saw a txt message from a number that is not in her contacts list. so i read it. It was very obsene and had alot of ual content on it and it was NOT from my father. then i txt back the number saying "u there or up?" and the person sent a pic message back of a ROSE. I then later had my BF call the number from his phone to find out what his name is. i found out his name. and there has only been those 4 txts to eachother that i know of. so im confused and hesitant to confront her about it. she might say "why are u checking my fone, its my business" i just dont know what to do