Assuming the power used was Summon Thought Servant DDI, it works exactly like a Heal check.
Opportunity Action: Triggered when a dying ally adjacent to the servant starts its turn; targets the triggering ally; the servant makes a Heal check to stabilize the target or to allow the target to use his or her second wind
In a Heal check, the subject is told to stop making death saving throws. There is no option to willingly make them. In addition, the subject is still likely unconscious and cannot take any actions.
Stabilize the Dying (DC 15): If the subject is dying, the subject stops making death saving throws until it takes damage. Being stabilized does not change the subject’s current hit point total.
Note that there is no stabilized condition and being stabilized does not remove the Dying condition. The character is no longer required to make Death Saving Throws.
This means that a stabilized character is still a valid target for effects like Energizing Strike DDI and Get Up! DDI
The Dying condition is removed from the character only when the character regains hit points. Being stabilized is not enough.