Has your girlfriend considered seeing a doctor about the pain she is feeling? Honestly, losing your virginity is slightly uncomfortable but it should not be unbearably painful especially if your girlfriend has already had sex once before. You may want to make that attempt by practicing sex more, being gentle, using lubricants if that helps. If she still feels that the pain is terrible, then she may want to be seen by a doctor to determine if she may have something called Vaginismus which can make sex very, very painful.
All the best,