my girlfriend married an illegal immigrant in Chester county pa would it be a legal marriage if we want to get married

There are 2 ways to end a marriage, divorce and annulment.   An annulment is having the marriage voided as if it has never occurred.  There are several reasons to grant an annulment such as already being married or fraud.  But it is much simpler, and probably cheaper, to obtain a divorce.  I always recommend using a lawyer for that as the rules can be confusing and it is generally affordable.