Step Dad Busted.....?

My mom has gone through a lot. She lost her mother about 1 year ago and aside from trying to take care of business (granted through power of attorney) concerning that in addition to her own household responsibilities, she's been on an emotional roller coaster. She and my step father have been through a lot but never anything as drastic as what I've discovered. "Someone" has been calling the house private and when me or her answers they hang up. While this looks suspicious, we really never gave it much thought. Considering my step father plays in a band, and works long weekends, my mom never investigated his whereabouts. They've been together for 12 years now, so she pretty much has him figured out. Last weekend, I was at a very expensive restaurant and saw him there with another woman! I didn't make myself known, I just sat back and observed. They didn't feed each other or anything like that, but they were looking very cozy, laughing and occasionally touching each others hand. My question is should I say something to my mom, considering we practically know all of his family and friends and this "female companion" definitely did not look familiar.