My husband and I are reaching a breaking point with our friend's poly partner

My husband (35 M) and I (47 M) have a friend (47M) that has, in the last 5 years, introduced us to their polyamorous partner (21 NB) who we will call Rose. Rose suffers from many mental disorders, was abused as a child, and me and my husband feel like they really need to develop a life of their own. But our friend has taken them in and their relationship has blended the border between romantic/sexual partner and child/dependent. This is one thing that has made us a little uncomfortable. These two always come as a pair, so there is really no avoiding the partner. This was ok for a while, and we were ok with hanging out with them, because we wanted to maintain our friendship. However, we are reaching a breaking point with the partner. We often dread any time we have to spend with them and are questioning ourselves whether or not the friendship is worth it. How can we proceed? What can/should we say to our friend, or do we just end the friendship?