The only people that Coulter could influence these days are the people who are already behind Trump. She has been irrelevant in national politics for quite some time, and is just an easy punching bag for the left. In fact, my guess is that ANY sort of relationship between Coulter and Trump will turn voters off, regardless of what she's saying.
Ann Coulter is only relevant as a liberal media outrage generator. She has no influence at all on conservative opinions and elections, compared to someone like William F Buckley or George Will. She is destined to go down in flames when the Trump campaign dies, and will only be remembered as a reference point for right wing ridicule.
Even my Republican friends tell me they watch Ann Coulter because she's so over the top that they find her funny. Not even people in her own party take her seriously, and that is going to magically change as the mouthpiece for Trump's immigration policy. She doesn't have enough credibility.
Very few people listen to Ann Coulter or ever hear her comments about issues. The only times she makes news is when she makes outrageously controversial statements. The mainstream media outlets will cover those comments, and then the story and Ann Coulter go away. Very few voters are aware she has made any statements recently.