Risk of Coronavirus 2 years after all clear

The link from the Mods is interesting but doesn't appear to specifically address what you are asking about. I think are two questions related to your mother's situation.

First, if someone is otherwise healthy and is two years out from active breast cancer treatment, are they at higher risk of either catching the disease or becoming seriously ill should they catch the disease?

Second, does Letrozole (and other AIs and Tamoxifen) affect the immune system in a way that could increase risk?

I may be wrong, but to my understanding, Letrozole is not considered "active treatment" and does not affect the immune system. Therefore on it's own, I don't believe that it would increase your mother's risk to catch COVID-19 or put her at higher risk of serious illness should she acquire the disease.

Did you mother have chemo and radiation? Those two treatments could reduce the ability of the immune system to react to an illness, but if your mother had few problems and has recovered well, I don't know if there would be any lingering impact on the immune system two years later,

How old is your mother? Being older does mean that one is at higher risk of becoming more seriously ill should you catch the disease.

Pretty sure there are no lasting impacts on immune system that far out of active treatment. The best we can do is follow the recommendations in the Moderator thread linked elsewhere (good diet, exercise, sleep, de-stress).