How noble of you to research for your friend! If your friend has been considering trying new things in bed, especially things geared more towards him than his partner, cockrings are actually a very good option.
These circular devices come in all shapes (fort of) and sizes. Mostly made of silicone, but sometimes metal, c-rings are meant to stimulate the wearer's orgasms, elongate his time, and as a result, help his partner (if female) reach orgasm as well. The way it works is simple. The ring is placed at the bottom of the penis to restrict blood circulation. The restriction then results in a bigger and harder erection. Sex therapist, Gigi Engle, does a great job at explaining why this happens:
Erections happen when arteries leading into the penis dilate, making the penis bigger than usual, while veins leading away from the penis constrict, trapping as much blood as possible in there. By adding another mode of constriction on top of that vein action, cock rings can help even more blood collect in the penis, resulting in more intense hard-ons.
A great option for beginners looking to experiment with this holy grail of erection is the Classix Performance Cock Ring Set. The Classix introduces you to the concept of wearing a ring without overbearing you with options.
Now, if you're a bit more experienced, or if you just want to go big your first time, I suggest using any of our vibrating options. The vibration will pleasure the both of you and almost ensure your partner (if female) reaches climax too. A great option is our Lelo Tor2 Vibrating Couples Cock Ring, but if you're on a slight budget, our Vibrating Clitoral Tongue Ring is another great option- she will be thanking you for days!
Regardless of the route you take, it is important that you do so safely. While I may not be a guy, I understand how sensitive your friend is and therefore, need to reiterate how important it is to try these toys cautiously. Be extra careful when putting it on, and even more so if you're feeling adventurous and trying one meant for your testicles as well. Don't be afraid to try to new things, simply be careful and smart about it.
Take your time applying sufficient water-based lube to put the ring on a flaccid or semi-erectile, place it at the bottom of the shaft (insert testicles through loop one at a time if using adequate toy), and enjoy! Remember to not keep the ring on longer than 20-30 minutes (read instructions!), and remove GENTLY.
Hope these tips helped or have inspired you... I mean your friend, to go on a head and explore new things ;) Always communicate with your partner, and have fun!