IF any of that happens, she just needs to stay calm. Make sure she has PLENTY of time at the airport so there is no need to rush at all.
As she sends DD through, make sure mom's getting eye contact with the TSA agent on the other side of the metal detector, so the agent knows that she is there. If they need to do something extra, they aren't going to just take the 5 year old away, but if she needs to come up with a plan of action, the action would be to project her voice (like in theater...it's not a *yell*, it's just a louder voice) and ask them to please wait for her.
Most likely nothing like that will happen, and daughter will just be standing there on the other side of the metal detector, waiting. If mom needs to be screened, she should simply take daughter's hand to make sure the agent knows that there's a child there.
I've seen the screening take place right there in the middle of everythign (unless the request for more privacy is made) so it's not like they will whisk one of them off. And in the small chance they do, if it happens, she needs to simply speak up clearly and politely.
I had a situation (even if one person on this forum doesn't believe me) where DH was being wanded (I could still see him, but it was probably 20 or 30 feet away), then I kept setting off the detector and they were about to take ME away too, and they were forgetting about my son standing there (either 3 or 4 years old). Things were moving a bit faster than I like (we had NO extra time b/c I had timed things really poorly) and they weren't hearing me... Thank goodness the lady behind me told me what was setting off the detector, a small and normal barrette, so I removed it, put it in a bin thing and sent it through the machine, and then I was able to walk through without setting it off again. Whew.
(the above story tends to bring out the questioners, so let me state....this day was absolutely an exception...someone had very obviously calibrated the detector very badly, as the barrette set it off, and the area that made the wands buzz on DH was his *zipper*, and that's what must have set off the detector in the first place...we've never encountered such a sensitive machine before, or after...no one should be worried about this same thing happening to them, and no one was angry or upset at the time, nor were either of us wearing ANYTHING else that set off the machines)
Anyway, just take it slow and easy, stay AWARE of what is going on, if something starts moving too fast, try to slow it down. Be polite, make sure the agents see that there is a mom and a daughter. It'll be OK.