Yes that is what the problem is.
Its been 3 months now minimum twice a week but with this situation you can say once a week lol
What you said is true. If he has a very large penis and is on top there's a chance he could hurt her. Most of the time things work out perfectly with couples. Sometimes there's a situation like this. If they really love each other they'll find a way to make things work with their sex life. This might mean that he will never be able to get more than half way in. He doesn't say how long they've been having sex. If it's only been a few times then there's a chance in the future that she may stretch enough during intercourse to accommodate him.
Tried all the things u mention but that did not help either..
I recommend the two of you have a lot of foreplay before intercourse. This will help with lubrication. I also recommend you buy a water based lubrication. If you're on the large side and your girlfriend is too dry this can cause a lot of pain. Be gentle and take your time with her. Make sure she's relaxed. Her being on top is a good position because she will be the one in control and this should make her less anxious about getting hurt.