Whether you mum has Tamoxifen would depend on whether she is post menopausal or not. The treatment for we older, post menopausal women is an aromatase inhibitor, which prevents the body from producing Oestrogen. I take Anastrazole and there is also Letrozole. But whatever she is prescribed, it will be after chemo, and is usually taken for at least 5 years. Tamoxifen is sometimes given for 10 years.
Sending hugs and wishing your mum all the best.