Eric might also use his position of authority. If he's in a position of power within the story's setting, he could simply order Peter to stop. For example, if Peter is violating some rules of the group or faction they belong to, Eric could threaten him with punishment if he doesn't let Tris go.
In this fanfiction, Harry's reaction could be a mix of fear and determination. He has faced many dangerous situations before, but being kidnapped is a new threat. He might initially be afraid, but then his Gryffindor courage kicks in. He could try to outwit Shield Tris. Maybe he pretends to be compliant at first, but then uses a clever spell or trick to turn the tables on her. He could also try to use his knowledge of the wizarding world and its hidden passages to evade her.
It could be her family. Her parents, if they are understanding, might step in to help with the baby so that Tris can still focus on her studies to some extent. Or perhaps an older sibling who has been through similar situations or is just very caring.