Grub doesn't boot Windows, it just chain loads it. Since os-prober is doing its job and providing an entry for WIndows 7 now, then the problem seems to be with Windows 7's boot loader.

To be on the safe side, from Linux:

sudo fdisk -l /dev/sda | grep type Expected output is Disklabel type: dos

(warning: udev names can change. Make sure /dev/sda is the device you want it to be by using the above command without the pipe and grep command or by using lsblk)

Next is to follow this guide on fixing the MBR and BCD.

You likely won't have to restore the boot sector. But if some reason you do end up going that route then to boot into Linux again you'll need to chroot in from the install medium and reinstall grub to the mbr.