One man (hos men). “This one,” demonstrative pronoun hos with men.
Hath faith (pisteuei). Like echei pistin (Act 14:9).
But he that is weak (ho de asthenōn). One would expect hos de (but that one) in contrast with hos men. Ho is demonstrative with de sometimes, but here is probably just the article with asthenōn.
Herbs (lachana). From lachanō, to dig. Hence garden herbs or vegetables. Denney feels certain that Paul has in mind a party of vegetarians in Rome.