She will find out.

Normally, that stuff happens. The stats are staggering as to how many men have had the pleasure of another woman (to put it poetically). And, normally, you can avoid the getting caught part if you keep enough separation between your wife and the girl you fucked. But, since she is your wife's friend, I'd start thinking of excuses and anything you can do to calm her down. As, she will find out. And she will be pissed. Unless, your wife finds it hot. Good luck.