First, let me say congratulations on your new baby!
Is this her first child? I ask because after the birth of my boy last year, I didn't expect to have such weak bladder control and leakage afterwards. I had an epidural as well (however, I had the catheter inserted right away), and I was told afterwards that you need to again build up your muscle that is weakened by the catheter itself. You are also more susceptible to getting a bladder infection after giving birth, if you have one inserted.
I'm not sure if this was really your question, but I just wanted to offer my experience, because it sounds like she's having some similar troubles! Tell her to do her kegals exercises, and that should help to strengthen that muscle again (although it may never be the same! I'm now realizing that mine is not as strong, now that I'm pregnant again with baby #2). If your really concerned about it though, I would definitely call her doctor, just to be sure it's normal. I would think it's more due to the catheter itself, rather than her holding in all that urine (by the way, 1.5 litres seems like a very large amount to me, as well).