I agree that it would be a good idea for her to see a gynecologist. She may be experiencing vaginismus
, an involuntary clenching of the muscles in the pubic area. However, the exam may not verify this: " Generally, when the attempt to put something in the vagina has ended, the muscles relax and return to normal. For this reason, medical examinations often fail to reveal any apparent problems unless the tightness occurs and is noted during the pelvic exam."
My mother reportedly had the same problem when she got married, as she was a virgin, and I suspect the teachings of her religion were involved with causing her involuntary muscle contractions. In a way, vaginismus is the female version of erectile dysfunction. It's not the woman's fault, and it's not under her control.
I wouldn't suggest just pushing through the resistance, as a painful experience is likely to cause the involuntary clenching to become worse through negative associations. I would suggest avoiding any vaginal penetration at all for awhile, and just focusing on all the other kinds of sexual pleasure you can give each other, such as all-over touching and kissing, manual sex (handjobs, and touching/rubbing the clitoris), oral sex, using external toys together (such as vibrators), and whatever else you can dream up. Be creative, and maybe read some erotica together for ideas. After you've gotten pretty good at making each other feel good in those ways, and she feels really ready after a lot of teasing, try using your finger or a very slim toy (like the Small Silk from Tantus, or a smooth plastic vibrator). If that works, meaning it feels good to her and isn't painful, then do that a few times, and then, next time you have sex, start with the finger or small toy, then move up to a slightly larger toy. Two fingers are usually fairly thick, so you might go from your pinky finger to your index or middle finger, rather than from one finger to two fingers.
Keep increasing the size of the penetrating item every few times you have sex, but starting off with the smallest item and working your way up through the different sizes. With patience and time, you'll hopefully graduate to a size close to your penis' size, and be able to have intercourse.
Try to remember, though, that there are many ways to have sex that don't involve a penis inside a vagina, and you'll be cutting down on your chances of pregnancy this way, too. Don't get frustrated if it takes a long time before she can even handle a small toy--just find other ways to get each other off!