My husbans left our home about 5 weeks ago. he filed for divorce in another county. how is this legal?

A case can be filed in any county that either party reside.  The only residency restriction is that the filer reside in PA for more than 6 months. You can challenge the venuw based on the circumstances and argue that the case would be better heard in the marital county.  But it is up to the judge to decide whether to move it. As a practical matter, a case can be resolved without any court appearance so it may not matter where it is heard.