They are rather pricey for us. And so far the events have been quite easy even without them. But that is probably just how it is for me since I've never been behind in our version. But then again, throwing them into summons can also be considered a waste when you get nothing but duplicate silvers. They could instead have been used to refill charisma and get your current team stronger.
Just saying that if he needs an extra boost to reach his goals in this event, the buffs are more likely to prove useful than taking a random chance at attempting to summon a strong new unit - who then still has to be leveled up and most likely even CC'd and leveled again to be useful. I've never done this event, and I don't know what his team looks like. I have no idea of the difficulty.