So basically this is a t, a client, and the client's ex.

The ex was never a client, right? So there isnt really a dual relationship. How does your friend even know about it, would be my question.

It seems to me, the t and the ex are free to engage in whatever, technically. Except that if i were the client, i would probably drop that t! If the ex were just doing construction work for the t, some temporary employment like that, or even housecleaning, it probably wouldnt bother me. Still, it is on the brink of a dual relationship and that to me could be unethical.