Hmmm,
Just thinking that her back passage is probably ok - I see a lot of problems from people who are overactive in the posterior pelvic floor.
Also, there is nothing to suggest that she has pelvic floor insufficiency - no incontinence. So in fact, her pelvic floor might be just fine but unable to cope with excessive intra-abdominal pressure.
If you really want to assess this lady, then glove up, put some KY jelly (without lidocaine!) on 2 fingers (middle and index) and insert into the vagina. Then have her use her pelvic floor muscles. Try to assess if she is pushing down into your hand or squeezing circumferentially etc.
An alternative method may be to get her to sit. Either you or her could then put the palm of the hand on the seat under the perineum - it would be like holding her genitals while she sat on your hand. Then ask her to use her pelvic floor muscles. Normal is a nice gradual lift away from your hand (try it now!!) Abnormal / increased intraabdominal pressure / "bearing down" would be pressure into your hand.
As i stated before, i am willing to bet she is excessively active in her abdominals and pushing that uterus out with excessive IAP.
If you choose to do the internal examination, i think it is only prudent to advise her EXACTLY what you are doing, why you are doing it and why it will help. That is why i don't do it - not only because i am a man (which really shouldn't have anything to do with it - there are loads of male OBGYNs - but because my wife wouldn't like it if i was doing these assessments! There are too many good physios around who do this sort of thing every day. I don't need to do it!).
Hope it helps