It is permissible for the couple to sleep in separate rooms. However, like you, we would discourage a kallah from planning to do so.

That halacha permits yichud even during niddah indicates that there are certain basic levels of intimacy that do continue even during niddah. In addition, this heter is founded upon an expectation that husband and wife will rise to the challenge of maintaining halacha even in their room at night alone. We suggest encouraging your kallah to rise to that challenge and not to give up on the intimacy of sharing a room when it is permitted. They could purchase twin beds, perhaps with a bed doubler if they are sensitive to the crack in between them.

It might help her for you to put the challenge of sexual temptation in perspective alongside the other temptations to sin that we experience and overcome and to present harchakot as the appropriate (and sufficient) tools developed by our Sages to overcome this particular challenge. Behatzlachah!