Hi,
You should not have to change the code manually there.
The current version of the group plugin doesn't use isAdmin. So if you have HikaShop 5.0.3, you should not have that error there.
And if you do, it means you have a permission issue in your plugins folders preventing Joomla from updating them when HikaShop is being updated. And thus, if other plugins of HikaShop are not being updated, you'll get many bugs and you'll spent a lot of time debugging things for nothing.
Instead, fix your permissions and install again the HikaShop install package so that it can automatically update all the plugins and you won't get these compatibility issues with the plugins anymore.