What can I do if I paid my friend’s rent, was too late, refused by landlord and is being evicted?

Unless the rental contract states, which I am sure it does not, that your friend has to pay the landlord in person, it does not matter at all who pays the rent. When your mother goes to a store to buy something you pleased on a lay away does the store care who pays for it? Of course not. All that is required is the physical offer to the landlord of the rent, where the landlord is able to take the offer. If that were not the situation, landlords could just refuse to accept money from their tenants and evict them. Moreover, if the landlord keeps the checks and it was a payment for nothing else than the rent, the landlord has been paid the rent, he just has not decided to deposit it into his bank account.