I've researched this topic a number of times over the years and have accumulated a long list of links to good resources, so I decided to share the best ones with him, and now with you.
This was my response:
Most people don't realize it, but an orgasm is something women have to learn to do. Or, really, it's something our nervous systems have to learn to do. Some girls are lucky and start masturbating when they're little, but a LOT of us start later and have to catch up. Because, as Drs. Julia Heiman and Joseph Lopiccolo pointed out 40 years ago in their groundbreaking book, Becoming Orgasmic, "the more orgasms a woman has had, the easier it is to have more."
The key to having orgasms more easily is having more orgasms. In other words, lots of practice. Your partner can help the process along by using a vibrator or a pulsating shower head to get several solo orgasms whenever she has a chance. And you can help by using your fingers, mouth, and tongue, and/or a vibrator to give her multiple orgasms before you start "normal" vaginal sex.
You can also incorporate a vibrator into vaginal sex. For example, either of you can hold it against her vulva and clit with one hand in doggy or cowgirl.
Aside from sheer experience, the next most important part of becoming more reliably orgasmic is reducing stress. As a starting point, I recommend reading Stress and Arousal:
A lot of us live high stress lives, and for most of us it doesn't take much tension to put on the brakes. If that's the case for you or your partner, then getting in the mood for sex isn't primarily about more romance and better foreplay. It's about reducing the stress and blocking out the distractions that are slamming on the brakes.As Dr. Emily Nagoski says,
If you’re the partner and want to generate desire, use your knowledge of her as an individual both to turn on the “ons” and turn off the “offs”: what makes her feel cared for, safe, affectionate, stress-free, sexy, and beautiful?Dr. Nagoski's book (see the list below) can also be very helpful, especially for ways to deal with "spectatoring" – thinking and worrying about what's going on during sex, particularly worrying about orgasms, which interferes with actually having them. Ironically, learning to be and feel in the present, and to enjoy the sensual pleasures of having sex while forgetting about orgasms entirely, can be crucial to becoming more orgasmic.
If your partner is a worrier or if she is dealing with a lot of tension, she can also benefit a great deal from learning two stress reduction techniques: Progressive muscle relaxation is simple – learn to clench and then relax major muscle groups in sequence. Meditation takes a bit longer to do, but it is also easy to learn and it is even more valuable. Many people have found these audio guides from Dartmouth College's website to be helpful with both meditation and PMR.
But if you really want to help your partner with this, the very best thing you can do is to learn the art of tantric massage. Forget all the spiritual stuff people are always trying to palm off as essential to tantric sex. At the core, tantric sex is a straightforward set of techniques that promote deep relaxation, extended arousal, and more intense orgasms.
To do either tantric massage or the kind of therapy outlined in Becoming Orgasmic, you really need a comfortable massage table. Amazon usually has a good selection of folding massage tables for under $90.
For more ideas, these are the best resources I've found for helping women become more orgasmic:
Orgasm Troubles – the Reddit r/sex FAQ
Dodson and Ross – Dr. Betty Dodson is over 80 and has been teaching women to have orgasms, and better orgasms, for 40+ years. The last time I looked, her website had 267 posts under How to Orgasm and 132 under How to Enhance Your Orgasms. I suggest starting here: Awakening the Clitoris and then checking out whatever looks interesting in those two categories.
First Orgasms, and How Women Vary – short, fun, and inspirational.
Becoming Orgasmic - a review and commentary on the next item.
Becoming Orgasmic: A Sexual and Personal Growth Program for Women, by Julia Heiman and Joseph Lopiccolo - the bible for pre-orgasmic women.
A Guide to Having Orgasms - Information for Pre-Orgasmic Women – this is an archived article, so you will need to use The Wayback Machine if you want to access the other articles in the sidebar.
Come as You Are: The Surprising New Science that Will Transform Your Sex Life by Dr. Emily Nagoski – the best overall guide to understanding women's sexuality.
The "dual control model" of arousal and desire – a cool cartoon presentation by Dr. Nagoski about the accelerator and the brakes.
She Comes First: The Thinking Man's Guide to Pleasuring a Woman, by Ian Kerner – the best book for men who care about satisfying women.
Better Sex 101 – an intro to both anatomy and functionality, double-checking to be sure you know the basics.
Finally, many women who are still having trouble with orgasms need a lot of stimulation, at least at the start, so it helps to have a good, strong vibrator to practice with. I prefer ball-headed vibrators like the Magic Wand or the Mystic Wand, but there are many good choices.
Good luck to both of you!