פּירוש אויף שמות 9:12

ולא שמע אליהם, “but he paid no heed to them.” [This was also a plague that had not been announced beforehand to Pharaoh. Ed.] An additional reason why Pharaoh paid no heed to this plague was that the boils proved responsive to medical treatment.

AND THE ETERNAL HARDENED THE HEART OF PHARAOH. It is possible that during the first plagues, the magicians hardened Pharaoh’s heart in order to pride themselves in their wisdom. But now that they did not come before him and there was none to help him and none to uphold him²⁰⁸See Isaiah 63:5. in his folly except his iniquities that ensnared him,²⁰⁹See Proverbs 5:22. [it was G-d Who hardened his heart]. It is possible that Scripture is alluding to that which our Rabbis have explained,²¹⁰Shemoth Rabbah 11:7. i.e., that during the first plagues, the hardening of Pharaoh’s heart was his own doing,²¹¹And not, as stated before, that it was caused by the magicians, who were proud of their arts. and now it was [rightfully] caused by G-d, as I have explained above.²¹²Above, 7:3. This is the true explanation.

.'ויחזק ה, G’d needed to reinforce Pharaoh’s heart as without this he would not have been able to endure this plague without his resistance collapsing. We know how Satan expected Job to react to the same affliction, from Job 2,5 “if You lay a hand on his bones and flesh he will surely blaspheme.”