Would ditto to other comments, it's great if your brave and can bounce back after falling off, but I would just make sure that the person you are helping knows what they are doing and is simply in need of a hand.
Starting can be very stressful for any youngster, so you need to make sure it is all done really carefully and quietly so as not to upset them. And don't forget to ask questions if your not sure or want to know more, as this will help you in the future if you are asked again
If he is already good on the ground to work with wearing a saddle and bridle, and can tolerate the strirrups flapping and the movement of the saddle it is likely he is ready to be backed. However, don't try and rush everything all at once, it can take several days or weeks to get them used to the sensation of having something weighted on their back that wouldn't normally be there. Naturally, horses are flight animals, and so anything that lands on their back in the wild would be wanting to eat them! This is why everything has to be done in stages
Once you and the owner feel happy that he is well and truly relaxed with the tack on him, take him into a safe, secure area i.e. paddock or sand school (keeps him safe and you if you land on your bottom!) Maybe lunge him or long line him (whichever her is used to) to get him focused on working, and to get a little freshness out of him before you get on. Make sure the stirrups are down so he knows what to expect next to his sides.
Then, slowly bring him to the centre of the school, and gently fuss over the saddle and rock it back and forth etc so he is desensitised to its presence. When he is calm, it might be easier to bring in a third person to hold him while you get sorted. Very slowly and carefully get a leg up and simply lie across his back, patting him gently on the off side with one of your hands to get him used to an even sensation. DO NOT sit astride him yet, simply let him stand feeling the sensation of having to hold your weight. If he is still relaxed, then ask whoever is holding him to gently walk forward just a few steps. If he starts to react then let him, and slowly come back together and try again. If he is still calm after a few steps, have him take a few more! Maybe just down one short side of the school. Dependant on his temperament this may be enough for him, but if he is still happy then slowly and carefully pull yourself upwards and sit astride him on the saddle. Lean your weight forward ever so slightly, and encourage him to take a step or two. If all goes well, this is plenty for one session, and gently dismount and make a huggggggeeeee fuss of him! The next time simply build up and build up until you can manage to mount him and sit astride him from the get go.
As long as everything is done nice and gently and calmly then you shouldn't have a problem. Make sure however he has had plenty done with him beforehand and hasn't just had a saddle whacked on him for the first time 5 mins before you sit on him. That's where the accidents happen! Hope that helps