Tagging a friend in a node
It is definitely possible, and some of the functionality you need already exists.

I would go the route of creating a new field widget for a standard user reference field that tackles limiting the possible list to current friends.

Have a look at user_relationship_views hook_menu implementation for an example of how you would do the user lookup (in this case we're looking up relationship types but you can get the idea). I think the code for doing this with standard user fields in views is pretty straightforward as well if my memory serves me correctly. To find all the relationships with the current user look at the api docs for user_relationships_load().

User relationships already comes with invite module integration (invite user x with y relationship attached to the invitation). The ui isn't that stellar but I'm guessing you could squeeze it into a node form with a little work.