Exactly I moved a lot during my childhood like at least once a year which was more difficult but even when I was young 6,7,8 I wasn't complaining because I knew it wasn't out of fun for my parents but necessity, they needed to have a place where they could afford the rent and many times landlords wanted to sell houses so we had to leave and sometimes if prices were too high that meant quite far away, new schools, new friends etc. But we adapted, became good at making new friends and going into mew environments, even now I'm someone who loves to move around and cannot imagine staying in one place more than a few years as I think I'd be bored which is likely due to my childhood. I moved consistently and went to dozens of schools (mainly primary but into early secondary as well), while it was sometimes difficult, you get on with it, and it was a choice between having a place to live or perhaps no place for us and we knew there wasn't the money she's 16, I understood this much younger and I'm sure she can now.
You can have empathy for her acknowledge it's slightly shit with her BF but that if she makes plans and they care about each they'll make it work. Also I think train travel to school will be fine I did it for 6th form, get into a routine of getting the early train, so if there's one at 8:20 but if that's delayed you'll be late, get into getting the one at 7:40, plus it gives you extra study time which for me when doing 4 A-levels was pretty valuable in the mornings when not many other people would be there and it was quite, so I got a lot done. Plus she's only doing 3 days, which yes is hard and and she'll have work to do outside of that, but I did 4 A-levels, 5 days a week and even with study periods there's stuff you need to do in your own time, if she plans out a good schedule she'll have loads of time for friends and not neglect her schooling. I'd get her to seriously consider a new college though, lots of people change college or 6th form to a place where they don't know people and there can be benefits of new teachers and new and different people as well. Is it slightly scary yes, but loads of people will be new and from different places, so she'll fit right in. If she doesn't want to do that, that's also fine, familiarity is nice as well. Honestly she'll be fine @Clarasmum444 it's a hard decision, she's sad so let her feel what she feels, perhaps she thought you may not actually do it as its just been a thought for 4 years whereas for me, it was so quick every time there wasn't time to argue. Be there for her and do what you need to do regardless which is move.