It will only roll on one way. The rolled side will be on the outside. Ask him to show you how if you are not sure. (If you happen to be a he, then practice by yourself.) Be sure to leave an inch or so free at the tip of his penis to allow space for the semen to come out. Adding a touch of lubricant to the head of his penis before rolling on the condom helps with his sensation - just don't apply too much and don't get any on the shaft, or the condom might slip off.
Don't worry. Practice makes perfect.
Oh, yeah, some guys will go soft if sex is started right away. Unless if falls off, it should be ok if he gets hard again. If if does fall off, get a new condom.
If you look at the condom when you remove it from its wrapper and look at the condom's rim you will see the rubber extend over the rim and down the inside on only one side. That's the side that goes against your penis.
If you add a little lubricant put it in the tip of the condom (only add a little) and make sure it is WATER SOLUABLE. Never use a petroleum based lubricant (like petroleum jelly) as it can cause the rubber to break down and tear.
I suggest you buy a few condoms and practice putting them on and taking them off. Also, if you masturbate while wearing them a few times you should get passed the erection problem.
Finally, if you ever put one on the wrong way (try and roll it on but it wont unroll because it's the wrong side) throw it out and use a new one. The risk is that you could get pre-ejaculate on what will then be the outside when you reverse it...a no, no!