Install a keylogger and get a VAR and snoop on his phone, you can install stuff on most to get texts and stuff. I would do it NOW. Don't tell him you have done so. Tell him you have a bad feeling about this woman and that you want him to stop contacting her, then see what he does. Based on his reactions so far, he'll blow you off. Then sit back and wait for the evidence. Once you have it, confront him and kick his sorry ass out.