I've often heard heal spurs and plantar Fasciitis called the same thing. (both hurt like he double hockey sticks - no matter how you look at it!)
Yes, crocs helped my PF tremendously! I've never had a comfortable WDW trip (foot wise) until I traveled with my Crocs.
Try a pair! It's not like they cost $100 or more! If they work for her SUPER! If not, you're not out a boatload of money.
If you have a trip planned, get them ahead of time. On the trip is not the time to test them.
Lots of people don't like the 'nubs' on the bottom of them. I admit the first time I put mine on without socks this spring I was shocked. I'd forgotten how they felt (wore them with socks all winter). But after about 15 minutes I didn't notice them anymore.