How do I convince my friend's mother to let her go shopping with me?

So here's the situation. I live in the Netherlands. Me and my friend G. want to go shopping in Hoofddorp, which is 152 km (194 miles) away from our home town. We want to go by train; train tickets cost us 14,95 euros (US$ 21). Now G.'s mom is really overprotective - she doesn't want G. to go out to bars or clubs, while that is very common for us here in Holland, and now G.s mom won't allow G. to go with me to Hoofddorp. We would be travelling during daytime and by train, which I think is completely safe - I've done it a lot of times. I'm 16 and G.'s 15. G comes from a Christian family, but absolutely nonstrict - they never attend church, they just read christian newspapers and sometimes the Bible and stuff. G. and I are both girls. What should I do - call G.s mother and explain that it is safe? Or let my mom call her? I don't know what to do anymore and I REALLY want to go on that shopping trip.