Dad cheating on mum??!?

[mirabelle]

what do you think of these convos i found on his facebook(yes i know i should look at his messages but this is serious!):

my dad:thats a sexy profile pic u got there.makes me wanna hold u xxx

the woman:aww tnx that was at a wedding in june.love u xx

qnd another convo:
my dad:i want the best for u and i will always find a place for you in my heart.love u

the woman:luv u too sweetie cant wait to see u on the 12th xxx

what do you think i should do?is my dad cheating on my mum??!

btw my parents are married and im a 13 year old girl.my mum is bipolar so i dont want to tell her because she would be very deppressed and ill.

no answers like:
"oh well u shouldnt have been snooping on your dads facebook"
or "tell your mum!"plz