In the state of Pennsylvania is the owner of a tree liable for root damage to a neighbor's sewer?

It actually depends upon what type of municipal ordinance (if any) is in place that pertains to the issue. When it pertains to sewer lines it is often not the tree roots that caused the original damage; but that the line was already ruptured and allowed roots to enter. The generaly consensus is of common law in which the land owner has no duty to those outside his property to correct natural conditions. There is also the legal presumption that the neighbor whose property has been infringed on has the option of "self-help" to correct the problem. This does not mean the person whose property has been damaged cannot pursue litigation, simply that it is unlikely they will be successful. The best option might be to speak with the owner of the property and try to come to an equitable agreement, perhaps sharing the cost of repairs.