He was devastated when he learnt that his best friend was migrating.
I would like to find out if devastated in this use case is a verb or an adjective.
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It's both. Or, it's either or.
There is really no way to syntactically say for sure in a sentence whether a passive participle is acting as a predicative subject complement or whether it is part of a passive verb construction.
Whether to interpret devastated here as an adjective or the matrix verb depends entirely on the semantics of the sentence.
If it's an adjective, the copula was is the main verb. It is usually (though by no means always) so that when you have a main clause and a subordinate clause, and the subordinate clause is introduced by when and describes a punctual action, the verb in the main clause sets the ‘scene’ for the action described in the subordinate clause. Example: “He was hungry when he came home” — ‘hungry’ describes the state he was currently and continually (non-momentarily) in at the single point in time when he arrived at his house. Interpreting your example like this, the person in question was in a state of being devastated (for some reason not specified) at the moment when he learnt that his friend was migrating. We might add an ‘already’ to make it clearer: “He was already devastated when he learnt that his friend was migrating, and this piece of bad news didn't improve his mood”.
If it's a passive verb construction, the time reference is usually (though not always) more punctual, and a following subordinate clause describing a punctual action is considered to be simultaneous with the main clause or immediately subsequent to it—it's not the backdrop against which the subordinate clause takes place. We can usually add a ‘by X’ clause to show the agent (the person doing whatever is being done to the subject) to make it clearer. For example, “He was overtaken by three cars when he slowed down”. If we interpret your example like this, we’re essentially saying that something devastated him at the time he learnt that his friend was migrating—presumably the news that his friend was migrating was what devastated him.
Semantically speaking, and devoid of context, it seems more likely that the second option is what is intended by the speaker/writer; but there is nothing syntactically in the sentence itself to show for sure that it isn't the first option, and in the right context, the first option may be more likely.