A close friend of mine is a Muslim and I would like to give her a M&M's Chocolate Candy as a souvenir as part of my USA trip. Can she still consume it even though the package has no Halal tag on it?

From the looks of it in doing some research it should be OK. Check the package for markings. Here are some references to go by:

This is the main part of it;
Allah says in Quran
“This day are (all) things are good and pure made lawful unto you. The Food of the people of the books is lawful to you and yours is lawful unto them” (Sura 5, Ayat 5).
If an authentic and genuine non meat kosher certified food products bearing kosher symbols such as OU, K. CRC, V, Kuf K, KVH, COR and Parve meet the Islamic dietary requirements, then those products are considered Halal.

They Key here is Kosher food.

Ref (1)
Halal Foods | Islam-Muslim (http://www.islam-muslim.net/tag/halal-foods/)

Ref (2)
http://www.thehersheycompany.com/brands/special-nutrition.aspx#/Kosher


Note M&Ms are not Kosher. But many candies are. You just have to check.