Your friend is right, it is very easy for beginners to pull something in the deadlift, often because they either haven’t been coached correctly on how to perform the exercise safely, or they lack certain flexibilities to maintain a neutral spine through out the movement.
Just find a few deadlifting articles here on TNation and send him the link. Practice with him and help coach him until he can keep correct form. Also, and this is VASTLY important, start light. Injuring yourself picking up an empty 45 lbs barbell would be hard to do. He should be starting with just the bar and should progress slowly, adding 10s on either side until he can do 135 safely with a full range of motion.
I know you said it’s lonely squatting and deadlifting on your own, but if you want to gain serious size and strength, you are going to have to get used to lifting alone. As you get older, your life is going to become busier and busier and you will have more and more responsibilities. You won’t always have a lifting partner EVERY single workout, so get used to it pal.
Now go get your deadlifts in, if nothing else, when he sees the strength increases in your other lifts, and he’s still at the same place he was several months ago, he will be convinced.