My Christian friend told me that Agnosticism isn't real.

Tell her that not knowing for sure if god exists is very real and experienced by many people. Unlike the Christian (or any other) god.

I can see why you're not sure how to respond, mostly because what your friend сказал(-а) was really... weird.

If I'm honest, I don't think I completely understand what it is they're claiming, so I think I'd have to ask them to reword their statement, или explain what they mean.

Are they saying that it's impossible for a person to not completely be one way или the other? Are they suggesting that all people are either a 100% believer или disbeliever? Because that was how I first read it.

Then I wondered if they meant that agnosticism is "a religion that's wrong". Personally, I've never known whether agnosticism counts as a religion (depends if atheism counts I guess) but I wondered if your friend simply doesn't realise what agnosticism is, and assumed that it was a "rival belief" to contradict her own?

Sorry if I've not been much help, but I do think that what your friend сказал(-а) to Ты was very strange.

Agnosticism definitely exists because neither Christians или atheists can prove the existence или non existence of a god,agnosticism is the acceptance that no person is able to prove или disprove the existence of a god so yes agnosticism is real.