By a completely inverse AG-groupoid we mean an inverse AG-groupoid A satisfying the identity xx−1 = x−1x, where x−1 denotes a unique element of A such that x = (xx−1)x and x−1 = (x−1x)x−1. We show that the set of all idempotents of such groupoid forms a semilattice and the Green's relations H,L,R,D and J coincide on A. The main result of this note says that any completely inverse AG-groupoid meets the famous Lallement's Lemma for regular semigroups. Finally, we show that the Green's relationH is both the least semilattice congruence and the maximum idempotent-separating congruence on any completely inverse AG-groupoid.