Dear Concernedstu,
Very sorry to hear your loved one is having another encounter with breast cancer.
The combination of Herceptin and Tamoxifen is OK as far I can see on a quick literature search. They appear to be synergistic - they enhance the other's effect. Below in a link to a prior thread on this site.
http://www.breastcancer.org/treatment/hormonal
The 2013 European Society of Medical Oncology guidelines indicate the two can be used together safely. It also depends on the medical condition of your loved one. Her oncologist will have taken this into consideration. The Taxotere is the drug with the hardest side effects but has strong effects on breast cancer.
Some people on the forum are having Perjeta added to Taxotere and Herceptin, and some have carbiplatin too (TCH-P regimen). Perjeta has been available since October in the USA but is not easily available in some other countries. I would definitely ask about getting it.
I have the impression that many of them are doing the treatment 3 weekly for neoadjuvant, but hopefully they'll pick up this thread and provide an answer for you. Some of the ladies with Stage 4 may be able to add their experience.
Your loved one is lucky to have you looking out for her.
Wishing you both the best
Aussie Girl