While in most cases I would agree with my colleague, each state does have its own laws regarding the revocation of a mediation agreement.
For example, in Texas, Section 6.602(b)(1) of the Family Code states:
(b) A mediated settlement agreement is binding on the parties if the agreement:
(1) provides, in a prominently displayed statement that is in boldfaced type or capital letters or underlined, that the agreement is not subject to revocation;
(2) is signed by each party to the agreement; and
(3) is signed by the party's attorney, if any, who is present at the time the agreement is signed.
So in Texas, a mediation agreement can be revoked by either party unless the agreement contains this very specific statement.
In other states, there will be a statement that permits revocation and without it the state laws prohibit revocation.
Each state establishes its own rules regarding the ability to revoke or challenge a signed mediation agreement.