In Fahrenheit 451, what alternative does Faber offer?
The above answer may well be the one you're looking for, but just in case I'll offer another one. Your question can be read on at least two levels.
Having met Faber before the text of the novel opens, Montag thinks of him when he has trouble comprehending what he reads. The two seem to understand one another at some level, as demonstrated by Faber giving Montag, a firemen, his address, and Montag keeping it and not turning Faber in. Once Montag has made a decision that there must be more to life, and books might be the answer, he calls Faber.
Faber, then, is an alternative way of life. Or rather, he is the gateway to another way of life. He can help Montag understand what he reads, and he gives Montag an ally.
That, too, in a sense, is what Faber offers Montag.
I am not sure if this is what you are asking (this is not a very specific question), but I think you are referring to the alternative that Faber gives Montag when Montag comes to his house for the first time.
When Montag comes, he has a plan to hide books in the homes of all the firemen so as to discredit them. But Faber does not think that is such a good idea. Instead, he tells Montag to take the "bullet." He will talk to Montag through the device and tell him what to say. Montag will be his ears and his voice. He hopes that he can undermine society that way.