لَا بَأْسَ طَهُورٌ إِنْ شَاءَ اللَّهُ
“There is no problem, If Allah wills, He will purge your sins by this illness.” (Bukhari)
And recite this dua seven times for his good health
أَسْأَلُ اللَّهَ الْعَظِيمَ رَبَّ الْعَرْشِ الْعَظِيمِ أَنْ يَشْفِيَكَ
“I ask Allah who is the Lofty and the Lord of the Mighty Throne that He cures you”
Prophet Mohammad (SAW.) said that by reciting this dua seven times, the ill person will regain his health and if death occurs then that (death) is a different issue (i.e. death is written it cannot be prevented). (Mishkat)