We appreciate your turning to us. Sleeping in the same bed during niddah is halachically prohibited, even when there is no easily available alternative.
If it is comfortable for a woman in this situation, it is permissible to mention her status to her hostess and ask if there is a discreet way she could bring a spare mattress or otherwise accommodate the couple. Alternatively, the couple could come up with a pretext to request extra blankets, and one of them could sleep on the floor.
Rav Shlomo Levi, in his “Shaarei Orah” (p. 11) permits a couple with no other option to place a large, heavy object as a barrier down the middle of the bed that would both create a visual division and prevent them from touching even in their sleep. It should be heavy enough to stay in place (and we would add that it should ideally also block motion from each spouse from being felt on the other spouse’s side of the bed). However, most halachic authorities do not permit this. Our site’s rabbinic supervisor, Rav Yehuda Henkin, would permit this approach only if neither spouse can spend the night on the floor (whether because of the room’s layout or for health reasons).
The optimal way to handle this situation is not to get into it. If a woman is in niddah (or suspects she will be soon) and the couple do not know their hosts or accommodations well, they could bring a sleeping bag, or even an inflatable air mattress, so that they are covered if necessary.
Please write back with any further questions.