The problem that I have with that is that in effect you would be asking her to stop doing what she thinks is best for her dc just for your convenience.
Whether the OPOL is the 'right' way of doing thing isn't the issue here. And whether you are right in saying she could speak english to her lo when she is with you.
The issue is you want her to do things your way because you prefer it that way.
Would you do the same thing for other areas of parenting?
FWIW, I did exactly what your friend did with my own dcs (and thankfully, none of my friends dump me because of it. They actually all told me how amazing it was and how lucky my dcs were to be bilingual). Now they are older, they still speak to my in mother tongue and they still to each other (when they want to) in their minority language. I have quite a few friends with a similar set up and they haven't achieved that. Their dcs are speaking to them only in english and are refusing to speak the minority language. But then they were also not as consistent in asking them to speak the minority language...
I accept that for some people, who has the opportunity to go back 'home, spend long period in the other country, they might be able to afford more flexibility. I don't think it's true for everyone.
As a side question, as a child with a parent was a foreigner, do you speak that language? Do you also feel you belong, in some ways, to your mum's country too (and that you are not 'just' english)?