My divorce was final in 2004. I was served with papers for modification of parenting plan and child support and his new wife (a lawyer) is representing him. Isn't this a conflict of interest?

No.

A conflict of interest occurs when one (such as an attorney) has two conflicting loyalties:

1. to one person (such as a client); and
2. to another person or entity (such as a husband).

In her case, loyalty #1 and #2 are the same, so their interests would not conflict.

There might be a problem, however, should she become a witness in the case. In general, an attorney may not also be a witness in the matter at hand.

I don't see any more of a conflict of interest then if he was representing himself. Which is none.