Of course, we cannot see into their hearts and souls. But I would suppose that, if they have same-sex attractions, this situation could be a near occasion of sin for them. That would be a concern, as well as scandal which someone else mentioned.
I have elderly family friends, women, who have lived together for nearly 50 years now. They are not Catholic, and we’ve never been certain of the nature of their relationship. They never really “gave scandal” in my mind because I knew them from childhood and their relationship at least appears totally a-sexual.
As a single woman, frankly, I can see myself entering a practical living relationship with a female friend in the future. If I am unmarried at age 65, for instance, I don’t see what would be wrong with buying a house with or renting to a dear girlfriend who is in similar circumstances…so we can help each other into old age. There doesn’t have to be anything sexual about a living arrangement.