I can think of a couple additional options, although one is merely an extension of another already mentioned. The catch, I believe, is that the come bet is officially “in action”, it belongs to neither the player nor the house. Consequently, the house could require the bet to be resolved by roll of the dice, and if the player did not want to, or for some reason could not wait for another shooter, the house could take down his contact information and send him his payoff if he won. This is the extension of the hardass option previously mentioned, and I doubt the house would enforce it unless the amount were sufficiently large to warrant the effort.
My actual third option would be to finish the roll without a pass line bet. A dealer or pit suit could do this, or the house could let the player do it. I think the requirement that the shooter have an interest in the point is a house rule, albeit a universal one AFAIK, to discourage shooters from leaving the table before the end of a hand.
I was once in a similar situation. I walked up to an empty table that had an established point of 8. I can’t remember now if there were any bets in action, but let’s assume not. Rather we’ll say that the shooter was allowed to throw the dice without a line bet, and then refused to make one and left when this mistake was noticed. As the lone player now I was given the dice and allowed to finish the hand without a line bet, although I did place the point. I was also told that I would be allowed to keep the dice after completing the hand for a hand of my own. I missed the 8 before any other players showed up, so this last condition became unnecessary. I can only assume, however, that had I made the 8 I would have been required to make a front or back line bet before continuing the departed player’s hand.