Is it a felony to knowingly open another person's mail? The
mail/letter was sent to "her" address however, was addressed to her
x-husband and his parents. This mail/letter was from a banking
institution. She opened this hoping for her own financial gain and
went as far as hiring an attorney. She wound up with egg on her face
however, was opening this mail, not addressed to her a felony?

Opening mail that is addressed to someone else is indeed a felony. 

From Title 18 of the U.S. Code:

"Section 1701. Obstruction of mail generally
Whoever knowingly and willfully obstructs or retards the passage of
the mail, or any carrier or conveyance carrying the mail, shall be
fined under this title or imprisoned not more than six months, or
both.

Section 1702. Obstruction of correspondence
Whoever takes any letter, postal card, or package out of any post
office or any authorized depository for mail matter, or from any
letter or mail carrier, or which has been in any post office or
authorized depository, or in the custody of any letter or mail
carrier, before it has been delivered to the person to whom it was
directed, with design to obstruct the correspondence, or to pry into
the business or secrets of another, or opens, secretes, embezzles, or
destroys the same, shall be fined under this title or imprisoned not
more than five years, or both.

Section 1703. Delay or destruction of mail or newspapers
(a) Whoever, being a Postal Service officer or employee, unlawfully
secretes, destroys, detains, delays, or opens any letter, postal card,
package, bag, or mail entrusted to them or which shall come into his
or her possession, and which was intended to be conveyed by mail, or
carried or delivered by any carrier or other employee of the Postal
Service, or forwarded through or delivered from any post office or
station thereof established by authority of the Postmaster General or
the Postal Service, shall be fined under this title or imprisoned not
more than five years, or both.

(b) Whoever, being a Postal Service officer or employee, improperly
detains, delays, or destroys any newspaper, or permits any other
person to detain, delay, or destroy the same, or opens, or permits any
other person to open, any mail or package of newspapers not directed
to the office of employment; or

Whoever, without authority, opens, or destroys any mail or package of
newspapers not directed to them, shall be fined under this title or
imprisoned not more than one year, or both."

United States Postal Inspection Service
http://www.usps.com/websites/depart/inspect/usc18/

The United States Postal Inspection Service is the law enforcement
agency which enforces federal laws concerning the U.S. Mail. To file a
complaint, the person to whom the mail was addressed should contact a
Postal Inspector. The nearest Postal Inspector can be located here:

United States Postal Inspection Service
http://www.usps.com/ncsc/locators/find-is.html

Not all complaints will necessarily result in charges being filed. I
once looked into filing a complaint against a family member who had
been opening my mail. The Postal Inspector's office told me quite
frankly that such complaints seldom result in fines or jail time,
unless it can be proven that fraud or theft was the motive, or actual
economic damages resulted from the incident. Penalties are more likely
to fall upon postal employees or business-related mail tamperers than
upon members of a private household.

Best regards,
pinkfreud

The answer is very thorough and exactly what I was looking for!!
Thankyou