over rule or not

[MikeStrybel]

Bob, kick this one around:

No one on, one out. Batter swings at the pitch and the ball goes down, rolling fifteen feet or so to where the F5 (playing in) grabs it in fair territory. PU IMMEDIATELY signals fair and takes a step or two towards the ball. F5 makes the throw to F3. The batter is hopping up and down on one foot and the HC is declaring that the ball was foul, off the batter's foot while in the box. You see the batter's reaction and had watched the ball hit the his foot, or at least the best you could see from 100+ feet away. Your PU gave an adamant signal IMMEDIATELY. Do you kill the play? Do you wait for him to ask your help? Agree with him or give what you had?

Admittedly, a foul is a quick, fairly easy call for a BU to assist on. However, this play involves a PU who IMMEDIATELY points fair (yes, waiting for a touch or stoppage is the correct mechanic but not in play here). Do you interject?