If he says that he loves her and she says she loves him and neither of them are lying or joking, he and she love each other. It is not possible to be mistaken about loving someone because loving is a linguistic and not material category. I love you is a performative utterance.
If I love you is a performative utterance and I am neither joking nor lying then when I tell you I love you it means I love you and is in fact proof that I love you and when you say that you love me it is the same thing.
Love exists and acts on the world insofar as it is called upon and believed in. Therefor love is God or a God or at least has the same ontologically provable status as God.