It would depend upon whether there was a conflict of interest.

Represent the spouse in regard to what?

First why is the husband, hiring his wife an attorney,

If the wife had legal action against her husband, she would be hiring her own attorney, he would not be furnishing her one.

So if he is hiring her an attorney to perhaps sue the spa, yes of course it can be someone from his own firm.

It would be the same as if the attorney sought to represent her himself. If he stands to gain from her, if the transaction or lawsuit or whatever it is works out in a certain way, there would be a conflict of interest.
If that were the case he couldn't represent her.
If he couldn't representer her, the attorney with his firm couldn't represent her.