So far, I don't think the paladin has done anything to seriously jeopardize her ethics or party. Does she know Ghandethar is evil? If not, then there is no reason to penalize her for associating with him. If she does, and they seem to be genuinely interested in each other, nothing says that she might not try to "bring him to the light". Also, love (or lust) can do strange things to people's heads, and paladins and evil overlords are people underneath all the bravado and combat prowess.
As for whether she betrayed her companions, I would have to say that she hasn't yet. She didn't tell them the whole truth, but she didn't outright lie to them either. As one of her companions, I would probably be worried if I knew she had lied and where she had been, but at this point the DM has no reason to smack her with penalties.
Also, another thing that is important is how active of a role the gods take in monitoring their followers. Do the gods monitor them closely for moral and ethical violations? If so, the gods don't really trust their followers, and likely only see them as pawns or at the best children. In such a case, the god would probably give her a little leeway, but after a certain point she would be stripped of powers.
On the other hand, if the gods do not monitor their followers extremely closely, it implies the gods have more important things to do than scrutinize the daily lives of their followers. In such a case, her actions might go unnoticed for quite some time, until she committed acts so contrary to her god's tenants that the god severed contact with her. This is more morally relative, and allows for priests of the gods who don't strictly follow that god's alignment, but instead sometimes let personal feelings guide their actions. Once she really started to stray, the god would likely send her subtle omens, and finally give her one more chance to repent or follower the darker path, and she would have to permanently live with whatever choice she made. To me, this is the more compelling possibility.