Yeah this is definitely hopium from the right. She’ll have signed some kind of sworn statement that has some sort of value to the prosecution or this deal wouldn’t be on. If she she says anything but what she’s already sworn to, she’ll have perjury charges, the plea deal will be off (since testifying truthfully is one of the terms), and it won’t really provide any help for Trump even if she decided to perjure herself out of loyalty. No jury would view that witness as credible. Judge would probably instruct the testimony to be disregarded or something too.