It's the proper thing to do, isn't it? For a couple to hold back with sex before her encounter with another man. If you're going to share her, then of course you want her to be tight and clean for him. I'm sure that she wants that too. But it can be hard to restrain oneself because the run up to an encounter is such an incredible turn on. So I agree that it is a good idea to then use a condom like
Discretefun66 does. And a month of abstinence makes sense too if she doesn't have so many encounters. Especially if it's along her full cycle, and he's the only one who unloaded in her. It may well be that you end up having to wait another three weeks: two weeks after her ovulation and a week during her period.
Anyway, it's quite something to discuss these things with the woman you love.