First of all, have her install Google Hangouts (free app) on her iPhone right now, before she leaves. She can use Hangouts to make free phone calls to US numbers, even landlines, while on WiFi. (She could call EU numbers too with Hangouts but not for free - like Skype, it would cost a few cents per minute, and she'd have to buy credit.). She should install Hangouts now and try it - sometimes Google asks you to verify your existing cell number in the US to prove you have cell service, but once you do that, you can use Hangouts to make calls even without a SIM card, on WiFi, or with a SIM card using its mobile data.
Second, install some kind of free texting app like WhatsApp or Google Voice (Google Voice gives you a free US phone number, and it will work with Hangouts too if she wants to receive free calls from the US on that number.).
Finally, she should plan on buying a European SIM card. She can wait until she gets to Belgium - might make sense, because then she'll have a Belgian number (might be nice for communicating with locals while she's in Europe).
I haven't bought a Belgian SIM, but here's some info about them:
EU roaming fees have mostly been abolished, but not every prepaid SIM even offers roaming - if it does, there will be no fee to roam in other European countries. She can just go to a mobile store in Belgium and have them set it all up for her. She may need her passport at the store to buy the SIM (many European countries require you to provide your identity to activate a SIM in their country - not sure about Belgium). Just make sure she asks them at the store about roaming.
If she has Sprint or T-Mobile now, they offer free unlimited roaming data overseas - but it's slow "2G" data. I just got back from Portugal and have Sprint, but I found the data too slow, so I topped up my Dutch Vodafone SIM from last year and use that instead - worked great. Some people find the "slow" 2G data acceptable to use.