"He sometimes drinks alcohol."
Translation:Il lui arrive de consommer de l'alcool.
I've never come across this type of construction.
I have just read on WordRef here that "Il arrive de " is an impersonal verbal expression that translates to "sometimes" and that it requires the indirect object or an indirect object pronoun to add clarity.
Example "Il nous arrive de prendre le métro pour aller au bureau "
"We sometimes take the metro to go to the office".
However, as "lui " is the indirect object pronoun for both "he" and "she", does that mean, lacking context, this sentence could also back translate to:
"She sometimes drinks alcohol"?
Yes, "il lui arrive de + infinitive" can translate to "he/she/it sometimes + conjugated verb".
You can also use "il arrive que" (no pronoun) to mean "it occurs that/it may happen", or again "sometimes/on occasions", etc.
"Qu'est-ce qui lui arrive/Qu'est-ce qu'il lui arrive ?" = What's the matter/wrong with him/her? What's happening to him/her?