I wonder why it is not possible to ask from a collaborator (or anybody) a permission to see his tree with the family group partly hidden. It woud help a lot at finding new connections. It woud not harm privacy very much, because a part of the family group is anyhow partly visible when starting from a common ancestor. Asking him to a common familygroup may be too much, if he is a very remote relative.
You can use the list filters (on the Relatives) tab to find public profiles managed by a specific collaborator. Type his/her name in the Focus Person field, select "Managed By" for the Group and under Filters, select "Public" in the privacy status drop-down. Scroll to the bottom and click the Update List button. Enjoy!