Read your bible and translations if you want to rely on it:

Adultery applies to men having sex with married or betrothed women.

And no, its not the other way around. Most major churches expanded the definition to reverse the roles over the last 700 years as women obtained cultural rights to have a say in the marriage process. Dont forget that women were considered actual property, and that several historical and biblical males had numerous, legal, God-sanctioned, sexual partners.

That aside.. .

I am guessing you and her friend have developed a mutual and legitimate love interest over the years, and your wife is either very aware and understanding of that, and/or out of true love for you.

Or, I fear she could be aware and too devastated to have sex, too...

This happens everywhere, everyday.

Talk it out openly, and be honest is my advice.