It's near impossible that Trump could get into power. If nothing else, he's entirely lacking in the necessary charisma.
In this hypothetical situation however, provided that Trump follows up on his xenophobia and racism and then dials it up to eleven, there's not much we could do. However, more likely is that Trump in power would not adversely affect minority populations. Muslims may not be Europe's favourite people right now, but they're still human. If mistreated, the EU would speak up. Like it or not, if the US wants to remain friendly with Europe, it has to avoid pissing us off too much. And believe me, considering that the US is still at odds with Russia, remaining friendly with Europe should be at the very top of its priorities.