How could a piece of glass in ones foot never cause a health problem? i know someone with this.

Assuming you : Are correct and the person has glass in the foot, the body is very complex. It tries to protect itself. So, the body will naturally try and protect itself from the foreign body and it may envelope the object by surrounding it in soft tissue. Think of it inits own little wrapping. It gets walled off. And potentially doesn't bother the person. Often times bullets don't get removed and stay in people.

Glass in foot: Glass is a bioinert material, which means that it doesn't provoke an inflammatory, foreign body response. A piece of glass has the potential to remain for years and years with no detrimental effects. Many other materials are similar and are intentionally put into the body to correct problems. Think of lens implants, artificial hips and artificial heart valves, to name a few.