Hmm ... I think Remus would freak out for a few seconds, try not to let it show, pull himself together and try to gently tell her that she's too good for him, that she deserves better. Then, if Tonks is stubborn, then Remus would also get a bit stubborn and angry, resulting in a fight, leaving Tonks deserted, upset, and crying.
Or maybe, at first, when she first fancies Remus, she tries to tell herself stuff like about that fairytale stuff people've mentioned, and she goes something like: "Merline, Tonks! You're in love with a werewolf! Werewolfs are the bad guys! You know that!" and stuff like that ... o;
Hope this's helped.