There are lounges you can use, but depending on where you are coming from/going to cause you to be ‘sitting around’ EWR is going to makes diffeerence in how easy it is to do so. From a ‘do you have access to the lounge’ standpoint, a J ticket on AC will allow you access into any of the UA clubs, or LH lounge for that matter, just not Polaris lounge.

The issue is that UA and AC in EWR are not well connected - the pier in Terminal A that AC uses is separate from the pier UA uses in A, and so you have to clear security in the UA pier to use their Club. Because the UA shuttle from C-A only stops at the UA pier, same issue with needing to use the one in C - though you could use the lounge in C, then clear security in the AC pier.

theoretically, a same day BP from any flight out of EWR should allow you to clear security in any terminal, but as always, YMMV.