If somebody has high heart rate (>90 at rest) it is very easy to hit the max hr during exercise even moderate. how safe is to exercise at that level?

Slowly increase...: Lack of conditioning can lead to a HR that increases rather quickly with exercise. Slowly increasing the duration & exertion with exercise - and being consistent with exercise will help lead to a lower resting HR and a slower increase. The max HR recommendations are safe; however, it isn't easy to maintain them for a length of time that would be beneficial. Build up your tolerance.