Legal Question in Civil Litigation in Pennsylvania

When you receive the subpoena, you would have to file a motion to quash it, but the grounds are very limited. See PA RCivP 234.4:

Rule 234.4. Subpoena. Notice to Attend. Notice to Produce. Relief from Compliance. Motion to Quash.

(a) The party serving a subpoena or a notice to attend or a notice to produce may excuse compliance therewith.

(b) A motion to quash a subpoena, notice to attend or notice to produce may be filed by a party, by the person served or by any other person with sufficient interest. After hearing, the court may make an order to protect a party, witness or other person from unreasonable annoyance, embarrassment, oppression, burden or expense.

While Its really up to the defendant and her legal counsel to decide if what you have to say is or is not relevant to the case.

I would talk to the defendant's lawyer, if she has one or the defendant, and explain that if the defendant proceeds to subpoena you, then you intend to tell the truth and that your testimony is not going to be very favorable and see if she still insists. If she is smart, she will not risk calling you because of the risk of unfavorable testimony.