I can definitely understand why you would want to calm down and take things slowly.
However, it sounds like your hubby and the lady in question are deep into a romantic relationship - that's very hard to just switch on and off on a whim (or a demand from someone else). It could lead to resentment building, which in the long run will make it even more difficult to get things sorted out and with added drama. She is probably feeling a lot of hurt because of her lost hubby, and can definitely use the people that she got used to relying on (and had fallen in love with).
I would say putting a moratorium on his other relationships should be an absolute last resort, because of the hurt that it will bring to both of them.
Do you feel that there could be a way for them to keep their relationship going, maybe in a modified form or something?
"Listen, or your tongue will make you deaf." - Native American Proverb