I'm in the USA; agree with Queuesarasarah that in general it could mean any of the things she listed. "We have to let you go" is a euphemism used by HR and management to fire someone OR to let someone know they've been "laid off" (made redundant). In this case, the fact that the position still exists and has been filled may be a sign that it's not "officially" a layoff; many states have laws that you can't (externally) advertise and hire for the same position within a certain number of months after a layoff. (You can move people internally to cover the position.)
While most states do have "at will" employment, there are various laws at the federal and state level that mitigate this. For example, it's illegal at the federal level to discriminate on the basis of race, color, religion, sex (including pregnancy, sexual orientation, or gender identity), national origin, age (40 or older), disability, or genetic information (including family medical history).
To avoid the appearance of illegal discrimination, which can lead to negative PR and costly lawsuits, most companies large enough to have an HR function will also have strict rules to document a firing "for cause". For example, an employee who comes in late several times a week may receive a verbal warning, a written warning, and a "final' written warning (which states the problem and resolution, and usually explicitly says that continuation of the problem will lead to termination of employment). There are certain things you can be fired for immediately, for example illegal activity done on company property or with company resources.
If the new manager wanted to "manage out" your colleague but didn't have any cause to fire her, it could have been structured as a layoff even if a layoff wouldn't normally be legal. In this case the employee typically agrees to leave and (importantly) NOT to contest the firing, not to discuss it publicly, and not to initiate any legal proceedings. In return, she may receive a monetary settlement, continued medical coverage (insurance) for up to a year, placement assistance to find a new job, an "eligible for rehire" status (important for future employment history/reference checks), and assurance that the company won't contest any claims for unemployment compensation (most states don't automatically pay benefits if someone is fired "for cause"). If that's the case then even if you asked, she wouldn't be able to tell you anything more about the circumstances.
Very generally, if your colleague is telling you she's been "let go" but she's still working until x date, it's likely structured as a layoff. If she'd been fired for cause she'd probably have been escorted off the premises/had all of her company resources such as email, phone, etc. shut down immediately. (In some cases that also happens with a layoff, although in that case she'd typically get paid for whatever the normal notice period would be.)