It shouldn't. It will go to Voicemail until she tops up again. I believe her Trac won't be getting anything till she tops up again. Voicemail, texts, etc.
Just to let her know, ANY cellphone, Tmo, Trac, etc, if it is in working condition, within range of a signal and is on can call 911 even when there are no minutes/money left in the account.
Also, ways for your friend to stretch her Tracfone minutes.
Buy a Double minutes for life card if she doesn't have it on her phone already. It automatically doubles the life of any Tracfone minutes she buys. Here is a site that has a tool with both promo codes which can earn you even more bonus minutes and a Double Minutes For Life tool to check if her phone has this feature. http://preprepaid.com/currenttracfonebonus.php The dmfl tool is under the Tracfone help tab settings.
Also, check out this Facebook App called Embee Mobile Wallet.
You answer surveys and earn free airtime credits redeemable on 20 different prepaid carriers including Tracfone and Tmobile. Use a separate email and Facebook account to avoid spam as sponsors may try to email you. Also, read the fine print to make sure you aren't being texted or signed up to a subscription charge you don't want (you can usually cancel at anytime if you text them a stop code. It is in the fine print of the offers that have this.). It is one way to get free airtime for your friends Safelink/Tracfone.
http://www.facebook.com/apps/application.php?id=58282125287