She lost a charity box that was in her care; should she replace it?

If this box was lost without any negligence or shortcoming on your part, then you do not have to replace it, but if you fell short in taking care of it, then you have to replace it, and replace the money that was in it, if there was any money in it, because you were entrusted with this box, and the trustee does not have to replace the item except if there is negligence.  

It is well known that there are two ways of acquiring a thing: as a trust, with limited responsibility, and as an owner with full responsibility. 

Acquiring with limited responsibility means acquiring an item or money on behalf of someone else, not taking possession of it, such as keeping something for someone else, borrowing something, renting something, or when one is a partner, working in a mudaarabah partnership, keeper of a waqf or executor of a will. 

Acquiring with full responsibility means acquiring something by taking possession of it, or to serve an interest of the one who acquires it, such as one who buys it, acquires it by means of a transaction, acquires it by way of a pledge, seizes it by force or takes possession of it. 

The ruling on acquiring something with limited responsibility is that the one who is in charge of this trust is not responsible for what is under his trust unless there is negligence or shortcoming on his part. 

The ruling on acquiring something with full responsibility is that the one who acquires the thing by taking possession of it or using it for his own benefit, is responsible for it in all cases, even if it is destroyed by a natural disaster or he is unable to return it to its owner; he is also responsible for it if it is destroyed. 

The one who takes something into his possession is fully responsible so long as it is in his possession. But if possession is transferred to another person by way of sale, or with his permission, like that which is given as a deposit, or without his permission, as when a thing is seized by force, in that case the one who has possession of it is responsible, even if possession is taken by a third party, by means of a transaction, deposit, or borrowing. The owner is still responsible. End quote. 

Based on this, so long as you used the box for the purpose for which it was intended, and you were not negligent or careless, then you do not have to replace it. If you do that and you bring another box or one that is better, then this is better for you and will help you avoid suspicion and gossip. It is a charity collection box and will not cost much, but you will be rewarded for it. So try to bring another box, but that is not binding upon you. 

And Allah knows best.