If two observers are moving away from each other at relativistic speeds, both observers would measure a slowing of time relative to the other observer. According to the theory of relativity, time dilation occurs for both observers in relative motion.
The key idea behind time dilation is that the flow of time appears to slow down for an observer who is moving relative to another observer. This means that each observer will perceive the other's clock to be ticking more slowly compared to their own. From each observer's perspective, they would see the other observer experiencing time dilation.
The principle of relativity states that the laws of physics are the same for all observers in uniform motion relative to each other. This means that each observer has an equally valid frame of reference, and therefore, both observers would measure time dilation relative to the other.
In summary, both observers would measure a slowing of time relative to each other when they are moving away from each other at relativistic speeds. This is a consequence of the relative motion between the observers and is consistent with the principles of relativity.