Luckily, I was playing yesterday against Durrani, so I have collected some data.
At T270, they had 6 Mountains and 5 Great Works of Writing in Amphitheatres, which resulted in +23 GW points.
The only "bug" I found is that Mountains give 2.25 GW points instead 1.5 (I have x1.5 twice). You can fix it in line 70 in DurraniCivRelated.lua (set from 1.5 to 1). Though maybe I did it intentionally so I don't know if it is "bug".
Anyway, your friend bonus 12 * 2.25 and 12 * 2 (assuming all great writings are in amphitheatre, though I except National Monument or something like that) = 51. +7 from base, 58, so he had probably a bit less, but +75% modifier should give him around 80.
12 GW is 7800 points? Great, so he needed around 100 turns of +80 bonus... Something is quite wrong. Let's say he got Great Writer from Globe Theatre. Maybe you played on quick time. Or maybe he got Pottery in Ruins, got Great Library and made Writing Guild around T50, started wasting population there instantly. Math doesn't check.

At this point I would like to say that it is Multiplayer Issue, but I unfortunately coded it too bulletproof. (However, I want you to note that playing my civs in Multi is extremely risky)
Now is the hard part, you have to enlighten me what is a huge balance issue. I think you just got a bit scared of big numbers. What is a Great Work? Phef.

The whole point of the civ was to encourage player to create Great Works of Literature instead waiting for and popping Cultural Boost.
Your friend needs now 1300 GW points (has 80 per turn), how it is related to 100 ~ 11 per turn. The point is that his total Culture isn't much higher than it would be if he would wait with Great Writers and he got some extra Tourism. Maybe the Culture distribution is an issue, because he got it earlier, so there is snowball effect.
UU - late game, my only modern one, irrevelant in this case, let's say it is decent.
UB - Amphitheatre, +1 Happiness, +2 GW points only IF the great work slot is filled. Humm, I would say it is vanilla balance, but it is stronger than Royal Library after all.
UA1 - Mountains, +1.5/2.25 GW points per Mountain in your territory. It is imbalanced in the same way as any terrain related bonus, from useless to overpowered. There is a map where 20% of land are mountains... Incas are OP. I should probably remove it then.
UA2 - Culture Bonus ~ this one I agree, is OP, but around T250, when you have a lot of culture per turn and you basically gain twice the Cultural Boost. I am probably gonna nerf it to "Gain Half of the Culture Boost bonus upon creating Great Work of Literature". I am gonna nerf it to dust.
UA: Gain Half of the Culture Boost bonus upon creating Great Work of Literature. Can build/
obtain (edit: meh

easy stuff) Observatory if there is Mountain nearby city.
UU: ?? will see, I don't like modern stuff
UB: Amphitheatre: +1 Happiness if slot of GWoL is filled. Provides GW points: +3 if Mountain adjacent, +2 if nearby, +1 otherwise.
I can replace some of the Culture bonus with Scientific one (observatory). Anyway, feel free to suggest what you feel is the huge imbalance and should be changed.

I actually like balance discussions, finding a bug is great too.