Hello KMD, and welcome to the forums! "Un fumeur malgré lui" certainly can - and since it's a "fumeur", almost certainly does - mean "passive smoker" as is clear from this (Canadian) context:
"1.2 Danger pour les non-fumeurs
Outre les maladies et les invalidités qui affligent les fumeurs, quiconque est exposé à la fumée de tabac présente dans le milieu devient un fumeur malgré lui et court le risque d'éprouver les mêmes problèmes de santé que le fumeur véritable." (santecom.qc.ca)
However it would be good to have a full sentence and the context in which you came across the expression. In another context, "malgré lui" could mean "unwilling" or "reluctant" or "unknowing" or "despite himself". Molière wrote a farce called 'Le Médecin malgré lui' in which a simple woodcutter is tricked into pretending that he is a doctor. Obviously you couldn't say "passive doctor" though.
Previous threads here, here and here discuss the sense of "malgré lui" in various contexts, so context is important. However, in the case of "fumeur", "passive smoker", as you suggest, is probably a good bet.