A friend just was served papers to get a blood test for paterity for an 8 yr child he did not know he may have fathered. The next step seems to be back child support depending on results. Any advice on legality.. how long can one wait to notify someone they may have a child?

This may be a 'which state?' question, but ordinarily a parent is not delinquent for an obligation he does not know about, even child support.

The only back support he might be responsible for is to reimburse the state for public assistance paid to the mother. Otherwise he would only be liable for ongoing support.

He really neeeds to consult an attorney

This depends on the law in the state where the parents live. In some states, the parent who was to receive child support can collect support owed even after the children are adults. The parent who was supposed to pay child support cannot claim that the child support is too old to be collected, or that the other parent should have tried to collect sooner, except to the extent collection is barred by the statute of limitations.

Statute of limitation-some states have from one to six year limits .