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.”