Have you met the co-worker? Would you be jealous of any co-worker? Do you have prophetic dreams? Let your intuition be your guide.
I don’t think you’re overreacting, though some may disagree.
When my DH was hiring secretaries I said, “She better be a grandma.” His secretary is over 60 years old! LOL! *Could* something happen? Of course! She’s a very good looking 60! So you see, there are no guarantees. LOL
If you trust him, and if you don’t get any weird vibe from her, it should be OK.