You can tell her that you have a cold and sleep together in case she has a cold, too. Or you can pretend that you can't sleep and like kicking bedding. Often, she won't sleep with you. Snoring can also drive her away. Or you can just tell her that you don't like sharing a bed with others. It may seem that you are heartless, but it's a good idea, and it won't hurt your's friendship, because you never like sleeping with others, not because of her relationship. Some things are more satisfying than the result you reluctantly accept. I hope it helps you.