Lemma 76.16.2. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of algebraic spaces over $S$. The following are equivalent:
$f$ is formally étale,
for every diagram
where $U$ and $V$ are schemes and the vertical arrows are étale the morphism of schemes $\psi $ is formally étale (as in More on Morphisms, Definition 37.8.1), and
for one such diagram with surjective vertical arrows the morphism $\psi $ is formally étale.
Proof. Assume $f$ is formally étale. By Lemma 76.13.5 the morphisms $U \to X$ and $V \to Y$ are formally étale. Thus by Lemma 76.13.3 the composition $U \to Y$ is formally étale. Then it follows from Lemma 76.13.8 that $U \to V$ is formally étale. Thus (1) implies (2). And (2) implies (3) trivially
Assume given a diagram as in (3). By Lemma 76.13.5 the morphism $V \to Y$ is formally étale. Thus by Lemma 76.13.3 the composition $U \to Y$ is formally étale. Then it follows from Lemma 76.13.6 that $X \to Y$ is formally étale, i.e., (1) holds. $\square$
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