This is actually pretty funny. You can understand what is generally considered one of the most difficult British accents to understand without problems, but struggle with others. Very strange. I guess it's simply a question of familiarisation with the way he speaks. The Geordie accent is unique, there is no doubt about that. You may also have to consider that he speaks more slowly to make it easier for you to understand, and I also wouldn't be surprised if he softens his accent as well.

You and your friend have, without realising it, become used to each other's English. You understand her accent and style of speaking more, and she knows what parts of English you don't know and therefore avoids that kind of vocabulary and grammar. This is a big problem, but it's really easy to solve: talk to as many people as possible, even strangers when you have the chance. Maybe a 2 minute conversation with a waiter might be more helpful than a 1 hour conversation with your Newcastle friend! This is hard for shy people, but if you're a language learner you have to force yourself to be brave and communicate with others. Good luck!

Eva... That... Is the most appalling Geordie accent I've ever heard in my life ... Seriously, that is up there with Dick van Dyke in Mary Poppins in terms of how bad it is. I can understand it, but dear me, what were the makers of that programme thinking? Cringeworthy.

https://m.youtube.com/watch?v=TOHLy8MVaCo

Three of the people in this clip have Geordie accents. The guy in the wooly hat doesn't, he has a Liverpool accent. 

Like Paul I was really amazed that you find the Newcastle/Geordie accent easier to understand than standard English. I'm a native speaker, but I understand maybe 20% of what these people are saying: https://www.youtube.com/watch?v=ZY4TT3VtR8o

My take is that it isn't strange at all, either you get e.g. Estuary English and can't pick up what is said in Geordie accent (this is me) or it's thw other way around: your friend taught you Geordie accent but not e.g. Estuary English.

Do you get what the guy says in this video clip? I don't!  https://youtu.be/Ei1DnFdJrww

For a short while I had a penpal who was a Geordie, but he only wrote never spoke so I didn't learn at all how to decipher that accent. Sounds like an entirely different language to me, like my native Swedish compared to Faroese or something. I'm still stuck with Estuary English as my favourite (used in my school when I first learned English "say after me"-style).