He has helped me when I was lonely.
Is this past tense ok?
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This is unremarkable in colloquial speech, but it is only marginally acceptable in formal registers.
Strictly speaking, you should not qualify a present perfect, which is a statement about the present, with temporal adjuncts which do not include the present; and when I was lonely refers exclusively to past occasions. You would do better to cast the when clause, too, in the present perfect. or to cast both in the simple past
He has helped me when I have been lonely.
He helped me when I was lonely.
The past-tense when clause is acceptable, however, if it is positioned as a ‘supplement’ or afterthought:
He has helped me—when I was lonely, and when I needed a friend.