There's a few different things you can say. You can ask them about any specific instances, to which they'll probably answer with the music festival or the POW Hamas took in the beginning.
To the first you can point out that video footage show IOF at the festival, shooting from the middle of a crowd. There's more, but I'm lazy, it's easy to find tho.
To the latter you can ask why Israel knowingly fired on their own people. This one you can also make more general - Human shields are vile because they make use of a "weakness" of the other, that they refuse to shoot civilians. When the IOF just shoots anyway, then how are they above critique?

You can also more generally counteract their narrative of human shields. Ask them for instances of Hamas making use of these, and discuss them with them. If their definition is that Hamas hides among civilians during bombings (wrong also, they hide in the tunnels) ask them: When Gaza is so tiny and so densely packed that any bombing will hot civilians, how can Hamas avoid being amidst civilians? How does this justify a carpet bombing of Gaza? Perhaps it is the IOFs methods that should be critiqued here, when they cannot avoid murdering civilians.

You can also go on the offensive. If you know they Support Ukraine, then challenge them on this. Show them news stories of Ukrainians making use of civilians as shields (amnesty International released a report on it). Ask if that justifies Russia's invasion. Ask if that justifies levelling Ukrainian cities.