You and your best friend have infants that were born very close together. So, they develop pretty much at the same rate, right? This may or may not be the case and it could be the case for some developmental milestones and not for others. That means that if your friends baby is already sitting up and rolling over and yours is not that you should not play the comparison game to determine if something is wrong with your child.
All babies develop at a different rate and while some of them will actually have developmental delays most of them are simply taking their own sweet time. Most babies learn to roll over somewhere between four and seven months, but that is just an average. Some babies are perfectly normal and dont begin rolling over until later.
The same goes for sitting. Your baby may begin sitting up on his own fairly early, or you may need to support his back with your hands or a pillow for some time. You can play with your child and help him develop quicker so he can sit up and roll over on his own. Add more tummy time and entertain him with toys and actions from above that make him want to move forward, stretch, and build his muscles. As you do this your child will be preparing for his next move, literally.
Of course, it is only ok that your infant is not sitting up or rolling over for a period of time. After a while there is reason to be concerned. Because of this you want to know when you should call your doctor and what you should be on the lookout for. If by four months of age your child is not rolling over or sitting up you should mention this to your doctor. He will let you know that the average age for these milestones is between four and seven months of age. However, your doctor will be aware of what is going on and may be able to give you some exercises and tips to follow. If by seven months of age your baby still has not met these achievements then make sure your doctor knows about it. He might want to see your child and decide if he has developmental delays or if he is just taking his time. As long as you keep your doctor informed then you should simply support your infant and do your best to help him grow and develop at his personal rate.