Perhaps you and the boys could leave at the same time as him sometimes and go to the park or something while they hang out together, that should at least put her mind at ease on that score.
As for the future, it sounds like the two of you need to put some time limits on how much time he can spend with a new person. He also needs to do some reading of his own on NRE. I highly recommend www.xeromag.com
for just such reading. I don't have time right now to do a full on search but he (Franklin Veaux, creator of that site) is very thorough.