Why should the boss feel responsible for his employee's debt? As long as he is making the trip truly voluntary, inviting spouses, and not counting this against their vaction time, no oine has the right to tell him what he must give to anyone. He is already providing them a salary in exchange for their services, which they are free to terminate at any time or to try to negotiate for a better deal.
It always bothers me when people look a gift horse in the mouth. Examples include someone receiving an unexpected gift and complaining that they don't frequent the business that the gift card is for, or that it isn't enough, or that they'd rather have cash than the card or than an opportunity to go to a picnic. The cash-for-picnic option would probably net them less than $15, but the point is, it's a gift. Something no one is obligated to give them, and which they are under no obligation to accept or to use once it is accepted.
If they don't want whatever is offered, politely say "no thank you", give it to someone else, or throw it away. In this case, go on the trip or tell the boss no thanks.