No, quarantine is not required for passengers who are only transiting the UK, regardless of having completed a COVID test. You will have to fill out a passenger locator form before you depart, although in my experience you won't be required to show it to anyone.
Source: UK Government website, and I did a similar journey last month
The UK government has a very extensive web page explaining how to self-isolate when arriving in the UK from abroad. With some caveats, your plan is doable, but perhaps not recommendable.
First of all, you are allowed to self-isolate staying with friends or family and as you already pointed out, you are allowed to leave the isolation for the purpose of ...
Will she have to do any test or provide any additional documents whatsoever?
Yes. https://www.iatatravelcentre.com/world.php -> France
A completed International Travel Certificate must be presented prior to boarding and to immigration upon arrival or when transiting France. The certificate can be obtained at consulates or at https://www.interieur.gouv....
AFAIK, if you are allowed in, and depending where you come from you have to stay home for 14 days on an honor system.
All french quotes are from the linked page from the French Consulate in Montréal (Canada).
When coming in, you will have to sign a declaration stating you have no covid symptoms.
"Une déclaration sur l’honneur que vous ne présentez pas ...
Under current exceptional circumstances, you need to check relevant COVID regulations as well.
You will be required to undergo COVID testing in India or France. A mandatory quarantine may be ordered if the result is positive or you present symptoms.
Additionally, you will need to sign a certificate that you are symptom-free and give reason for your travel to ...