It has nothing to do with age, @Imadethisupwithnoforethought. Not my age, not his age or his girlfriend’s age. (Except that the younger they are, and the younger the relationship is, the more strained things will eventually be in the home and in the office.)
In any case “secretary” hasn’t been “a good job” for decades, and it’s not getting any better. Help the girlfriend get another job, somewhere else, with better prospects for advancement.
However, I did qualify my earlier answer, too: If they’ve already had and survived some good fights and a fair amount of conflict in the relationship, and it’s not a “blossoming romance”, then it might not be a bad move… if she’s willing to settle for “secretary”. And if the rest of the office staff are willing to have a mole in their midst.
It’s generally not a good idea, for the relationship, for the give-and-take between boss and secretary, and for morale in the rest of the office, unless both of these parties are angels.