You think Mom is cheating. Do you confront her or tell Dad?

You think Mom is cheating. Do you confront her or tell Dad?

"I was out with my boyfriend on Memorial Day a few towns away from where I live and I saw my mom at dinner with another guy. My dad was at a BBQ. When I asked my mom what she was up to while we were gone, she said she was just running some errands.
Should I confront my mom about this or tell my dad? I don’t know what to do."