Hi, welcome to the forums!
She will definitely need a visa; it's a question whether to apply for one in advance before travelling, or showing up at the border and getting one there after speaking to the immigration officer (IO). For most people, it's the latter. They land at Heathrow, have a 5-minute chat with the IO about their intentions visiting the UK, and if the IO is satisfied they aren't an immigration risk, gives them a stamp in their passport that admits them for up to 6 months.
Generally speaking, you don't need a visa in advance unless you have previous adverse UK immigration history (like a previous refused entry, or previously refused visa application). You can apply for one in advance if you wish for the peace of mind, but it's not necessary.
She'll need to be able to demonstrate strong ties to the US when she arrives to be granted entry. The IO must be satisfied that she's visiting the UK and fully intends to return to the US at the end of her visit. She must be able to show that she has things to return to in the US like a job, home to take care of, etc. She'll need to show she can financially support herself throughout her stay, can pay for any NHS treatment if needed, and will not be working in any capacity (no volunteering, no working remotely online - nothing). She'll have to show she has a place to stay - if with you, it would be helpful to write a letter of support stating so.
The longer she intends to stay, the more ties she must demonstrate to the US. Someone staying for 3 weeks will encounter less scrutiny at the border than someone proposing to stay 3 months.
