I totally agree about the teen relationships above. Of course it's highly unlikely the person she's with at 15 will a long term partner as an adult - I have a teen myself and I know how their relationships are. But it needs to be let to break up/fizzle out in as normal a way as possible (accepting this is very difficult for all concerned) with support for all, and without pressure from his parents to make their son happy OR pressure from her parents to avoid the DD having to engage with the realities of cancer.
I would just say though, one thing no one has realised here is that young cancer patients going through treatment don't necessarily have much of their time available for phone calls.
You or your daughter might be experiencing texts from his parents as emotional pressure to call and make him happy, but the parents might be experiencing them as "oh great, finally an hour where he's awake, alert, not in theatre, not sedated, not vomiting, not having a consultation with a specialist, not having his counselling, etc etc, maybe one of his friends could call now". It might actually be more of a logistical thing from their point of view.
I second the poster above who asked you to really engage on the question of what the "pressure" looks like, whether your daughter is experiencing anything as "pressure", and whether you possibly want to see "pressure" so you can tell your daughter to disengage and spare all of you from having to acknowledge cancer happens, a lot, to a lot of people.
Is it one or two texts along the lines of the above, or a barrage of constant messages full of emotional requests or accusations etc? There is a very big difference.
None of this is easy for anyone, and there are no templates for negotiating these situations, so sending love and reassurance to you as well as both the teens and the other parents.