You can use Facets to add a extra value to that edge relation.
https://docs.dgraph.io/query-language/#facets-edge-attributes
{
set {
_:alice <friend> _:bob (since=2018-02-02T13:01:09, close=true, relative=false) .
_:alice <friend> _:charlie (since=2018-02-02T13:01:09, close=false, relative=true) .
_:alice <friend> _:dave (since=2018-02-02T13:01:09, close=true, relative=true) .
}
}