she's trying too hard tell her to relax and enjoy the moment,the harder she tries the less chance it will happen.suck on her toes as part of the foreplay,get them nice and wet and roll your tongue over under and between each one.that will get her on the right track
Just a suggestion...Have you guys tried using "sex toys" I have the best orgasms when my boyfriend uses a dildo while performing oral sex on me. Or just uses his fingers inside my vagina while giving me oral sex. Start out slow and take your time with her. It could really take a while. Also I have read and Ive tried that if you focus on your breathing during stimultation it will bring you to orgasm faster, and the orgasm will be stronger. I agree also with using a vibrator on the clitoris. the best ones are a little bulky but work. The standard back massager/wand massager with the bulb type end on it. Have her lie on her tummy, or on her knees, enter her from behind and let her use the massager on her clitoris while having sex that way....GUARANTEED!!!! Hope that works for you guys! Take care friend.
Is it possible that she overstimulated herself? years ago she used to masturbate very frequently.
She needs to relax and NOT focus so much on having the orgasm, she needs to focus on what is happening when it's happening and what she's feeling and stop worrying about when/if she is going to orgasm. I know that's very hard to do but she should focus on what's happening in the now moment. Also, most women can NOT orgasm with just vaginal intercourse alone, they also need clitoral stimulation. She could try a small vibrator during intercourse, you or she could stimulate her clitoris during intercourse, you could focus on oral and try and get her to orgasm first and then have intercourse for your own orgasm. She should try experimenting either with you or by herself to figure out what she likes best.
Just to add my own experience, i didn't have an orgasm until 5 years after I was sexually active.
Sorry that was a bit graphic but i figured it would help. There are also many many books to help with female orgasms.