The rules haven’t changed yet.
Whilst she may have both vaccine doses it doesn’t 100% guarantee she won’t get I’ll but most likely relatively minor if she does; and it definitely doesn’t guarantee she won’t pass it on even if she doesn’t get symptoms. None of the tests are 100% at picking up infection – largely because the swabbing process is variable.
I believe the logic behind the planned rule change in August is that by then all adults who wish* to be vaccinated will have had both doses plus the time for second dose to work.
Lots of people are ignoring the self isolation instructions/advice. That doesn’t make it right; but if she/you live alone presumably you are bubbled together. Given the gov want to pass the responsibility for being sensible to us you might want to consider: if you go visit have you been vaccinated? Both doses? More than 14 days ago? That would significantly reduce your personal risk.
Meeting in her garden would be safer for you than close contact indoors.
If you go visit assume you will pick it up as an asymptomatic carrier – who else will you expose? If you work from home, get shopping before you go etc potentially virtually nobody. If you are a teacher potentially dozens of kids who each will expose a handful more people who are potentially vulnerable. If I was going I would do lateral flow tests every two days for the next two weeks – but always assume a negative tells you nothing, only a positive result should change your behaviour.
*this does ignore a very small number of people who for clinical reasons cannot be vaccinated, and who are likely particularly vulnerable to covid too, but Boris isn’t one for fine detail!