Even though two budgies of the same sex will get along very well in many cases, they are all individuals, and some might not like each other too well. Sounds like the newer guy is dominant, and has bullied the older one in his own home!
I also need to mention here that it is never a good idea to introduce another budgie to an existing one without having been in quarantine for 30-45 days. https://www.talkbudgies.com/new-budgi...necessary.html
This should have been done, but if it wasn’t and you don’t notice signs of illness in either bird, then let’s move on...
The first budgie is evidently not happy with the new arrangement, and it’s showing in his behavior. Stress isn’t healthy for your budgie.
A good arrangement would be to house both of your budgies separately, so they have their own space, food, toys, etc. You’ll need to get your new budgie his own pad. 18”x18”x30” minimum. You can place the cages side by side. The existing budgie should of course have the cage he’s always been accustomed to. After things have settled again with this new arrangement, your girlfriend can try bonding again with the older budgie.
Please have your girlfriend join the forum. We cannot answer any more questions by a third party. Thanks for understanding