If you're a Christian who's been reluctant to vote for Donald Trump because of his past - shall we say - moral shortcomings, a secondhand report of his possible salvation is certainly not a reason all by itself to change that position. And as we've discussed in the past, it's entirely possible God could be using Trump regardless of his faith or lack thereof.
But one thing any Christian should recognize is that a person's past is cleansed once they come to Christ, so if Dr. James Dobson is accurately reporting what he says happened recently, what do you do with that? There's no embed code available, but here's where you can hear the audio interview in which Dobson makes the claim.