Yes, it's an error there is no mention of 'ball hitting glove' in the rule book. A frequent comment is, at least the kid who got a glove on it, got close enough to get a glove on it. You can find the rules at
OBR Scoring RulesThe exerpt to show your buddy is below
Per Official Baseball Rules (OBR):
10.12 ERRORS
An error is a statistic charged against a fielder whose action has assisted the team on offense, as set forth in this Rule 10.12.
(a) The official scorer shall charge an error against any fielder:
(1) whose misplay (fumble, muff or wild throw) prolongs the time at bat of a batter, prolongs the presence on the bases of a runner or permits a runner to advance one or more bases, unless, in the judgment of the official scorer, such fielder deliberately permits a foul fly to fall safe with a runner on third base before two are out in order that the runner on third shall not score after the catch;
Rule 10.12
109
Rule 10.12(a)(1) Comment: ... The official scorer shall charge an outfielder with an error if such outfielder allows a fly ball to drop to the ground if, in the official scorer’s judgment, an outfielder at that position making ordinary effort would have caught such fly ball...