Ask that it be placed in a corner to avoid accidental bumps and falls. Loosely cover it with your harp case or a dust cover – do not enclose it tightly as you want it to breath and not trap humidity. As for loosening the strings, that I think will depend on how long you expect it to be there and what kind of string you have.
In general I recommend not loosening them as repeated loosening and tightening is not good for the board; but if you expect to be away for six months or more it might be a good idea. Harps are happiest when they are played but if that is not possible for extended periods probably best to relieve a bit of stress.
Biagio