Babies get weird rashes. There have been a few times when Jack has had a random rash pop up and we never determine the cause, but it just goes away in a day or two. Right now we have a weird situation where he breaks out in a weird rash on his trunk only (it's not pimply, and it's not hives, it's just a weird series of reddish bumps) every day around mid-morning. It's gone by bedtime. I've tried everything: changing up what he eats for breakfast, switching laundry detergents, etc, and we can not figure out what's causing it. And it is never there when he wakes up in the morning, it happens during the day. So weird.
Regarding your friend's baby, what other info did she give you about the rash (probably none, bc it was a voicemail, right?) My understanding is that rashes related to a food allergy either appear in the form of eczema or affect the entire body. It's totally possible that it is from something she ate, though.