(answer in bold, the rest is just my yakking)
hmm.. best guess? bicep curls.
If he's doing a bicep curl and gets caught short, he can drop the dumbbell onto the weights mattress without too much trouble. Though you still can have a spotter stand in front of him with hands under the curl bar.
In general, exercises should always be spotted if they require a weight to be passed over the head (such as the bench press) or if they require a weight to be placed at the neck or shoulder (such as the squat).
In fact, simply having a spotter is not necessarily enough to ensure weightlifting activities are done safely; the spotter also needs to be familiar with their role and be able to assist enough with the weights being lifted. The main goal of a spotter is to keep the weightlifter safe (ensure proper technique, protection from dropping, protection from straining) and prevent them from injuring themselves or others, and perhaps to protect the equipment too. A spotter can also be your 'cheerleader', a source of motivation, confidence, and improvement.
To do without a spotter, it really depends on your level of experience, how much relative weight you are lifting, and where you are in the day's workout (energy depletion).
Hopefully Ben did not increase his weights without a spotter. For now, just a verbal reminder that I would prefer him to have a spotter (even better if it's me and not some friend), no matter how safe he believes it to be.
Side note, never spot anyone doing power exercises as it can be dangerous to both you and the lifter...