Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
Everything is written and decreed by Allah, the Exalted. This is indicated by His saying:
﴿مَا فَرَّطْنَا فِي الْكِتَابِ مِن شَيْءٍ﴾
“We have neglected nothing in the Book.”
And His saying:
﴿وَكُلُّ شَيْءٍ فَعَلُوهُ فِي الزُّبُرِ وَكُلُّ صَغِيرٍ وَكَبِيرٍ مُسْتَطَرٌ﴾
“And everything they have done is recorded in the Scriptures, and every small and great thing is inscribed.”
It is also mentioned in the hadith that Allah, the Exalted, decreed the destinies of all creatures before creating the heavens and the earth.
However, this decree is part of the unseen, which we do not know before it occurs. Therefore, a person may not use divine decree as an excuse to abandon action and the means available to him. Rather, Allah, the Exalted, has commanded us to take the appropriate means, including seeking medical treatment. The Prophet ﷺ said:
“O servants of Allah, seek treatment, and do not seek treatment with anything unlawful, for Allah has not sent down any disease without also sending down its cure.”
And he ﷺ said:
“Act, for everyone will find it easy to do that for which he was created.”
Thus, a person has will, ability, choice, and intention, but none of these exist outside the power and will of Allah. We do not know what has been written for us until it occurs. Therefore, we must take the lawful means and then be content with whatever Allah, the Exalted, decrees.
Accordingly, there is no contradiction between trying to treat a sick person and save him from death and believing in divine decree and destiny. Treatment itself is one of the means that Allah has decreed. The patient's recovery may have been decreed through this treatment, while his death may also have been decreed despite taking all available means.
The correct approach is for the Muslim to combine belief in divine decree with taking the appropriate means, and not to abandon treatment on the pretext that death or recovery has already been written.
And Allah knows best.