Pop unders are a guaranteed way to get most users to turn on their adblock script against your site and/or disable javascript on your pages, if they succeed at all. Browsers ship with pop up blockers for a reason, users find them incredibly obnoxious and rude. You will do more harm to your brand with this technique than help.
I totally agree with @Michael_Morris and @Paul_Wilkins on this one. I was going to reply, earlier, stating that such an underhanded action doesn’t deserve help, but I think Michael and Paul have provided the best response.
While that’s my knee jerk response as well, we don’t know if the OP has already urged the client not to do this and has given up and is going ahead with it just to placate them. In that event we’d just be preaching to the choir. Since what he wants to do is ultimately not harmful to someone else’s machine (just hella annoying) I provided the how of it.