Place potatoes in a large saucepan; add water to cover by 2 inches. Bring to a boil over medium-high; cook until potatoes are very tender, 13 to 14 minutes. Drain and return to saucepan. Add milk and salt; mash with a potato masher until smooth.
Pulse together parsley, cilantro, oil, vinegar, and garlic in a food processor until very finely chopped. Transfer mixture to a small bowl, and stir in softened butter until combined. Measure 3 Tbs. herb-butter mixture into a small bowl; stir remaining herb-butter mixture into mashed potatoes in saucepan. Transfer mashed potatoes to a serving bowl. Swirl reserved herb-butter mixture into top of mashed potatoes. If desired, sprinkle with crushed red pepper.
Serve him plain mashed potatoes and let him swirl in as little or as much herb butter as he'd like.