Key of C diatonic harps (like yours) are tuned so that the blow notes, starting at hole 1, are:
C E G C E G C E G C
So one thing to try first is to take her tuner and see if the rest of the blow notes are producing the notes they should. The blow notes from holes 1,4,7, and 10 should all be C. If it turns out that all the other ones produce a C note when blown, then your problem is just the 4 blow reed. In all likelihood, it has probably gone flat, though it’s quite rare for a reed to go flat an entire step. Usually when reeds go flat, they’ll get to about 1/2 step flat at the most before they fatigue to the point they actually break.
The other thing to address is if while using the tuner you find that the other bloIw hole notes I mentioned also don’t check out, then there’s a different problem entirely. For one, someone could have retuned the harp at some point to suit their needs, which would change the notes produced. Another possibility would be that the cover plate with the C engraved is not original to the harmonica. It could have been replaced in the past, which would make the engraving of C on the cover plate inaccurate.
Report back with the notes you get according to the tuner. Also, Lee Oskar’s website has a page that has breakdowns of harmonicas in each key and the notes that each hole produces, including bends. Very helpful when tuning or diagnosing a bad reed.
Hope this helps, feel free to ask more questions if you’re confused about anything.