Does anal sex ruin the anus to where you can't control movements?
Hi there. Your friend is half right.
The anus (back passage, arsehole) has a sphincter muscle to do its job of letting bowel contents out when we want it to. The sphincter is like a ring of muscle that can open and close. So its job is important to prevent bowel accidents and mess.
In anal sex the sphincter can be damaged or torn if people are too rough. Anal sex does need some gentleness, patience and lubrication to relax the partner and open up the sphincter. Unlike the vagina, which is reasonably tough (it fits babies through) and has its own lubrication, the anus doesn't have much lubrication of its own.
Anal sex, as long as it's done properly and gently, shouldn't ruin the anus. But it's true that the anus wasn't built for sex to the degree that the vagina was.