Well it sounds an odd story either way!
I've sat in on tribunals and I find it hard to believe that a pair of crutches would sway them I'm afraid. And higher rate mob is the one they particularly watch for with fraud and crutches in no way get you through. In fact, crutches would be a sign that she probably does not fit the criteria as they imply an activity level beyond the criteria.
Your story is confusing, as i believe that alot of disability is hidden from the outside world and it's very sad that people feel they have a right to decide how ill or disabled a person is without asking them, we don't have it displayed for the world to see you know.
On the other side, going hiking is a step too far in that direction if it really was full on hiking? Even with a variable condition, someone wouldn't be that well for a sustained period of time and be able to claim that rate dla. However I think some of my more distant colleagues would say the same about me as they see me for 3 hrs weekly whilst I'm lying on a sofa but going to the loo and to get a drink ok... However it's only about 4 meters away so I can put up a pretense of normality for that long, and they don't see the 48 hrs of pain that comes after the weekly performance.
Mostly, I do find it hard to believe that a professional couple on their wages would risk all for the small amount if dla money given. I do see that a solicitor might be uniquely qualified to be able to defraud the system (access to case law etc), however, they have so little reason to and a huge incentive not to.
The way people go on about dla makes it sound like its a free ticket to a life of no work and many holidays a year. It's really not, it makes the difference between surviving and not surviving for disabled people. It's really hard to get and the fraud rate is the lowest of any benefit.
Conclusion: I think you should talk to her about her illness and ask her how it is nowadays... Maybe she doesn't mention it because it makes her feel down admitting how bad it is, or maybe it's private, or maybe she knows it's a conversation killer (believe me it really is!), and she d rather talk about things a well and healthy person can relate to. Or maybe she's a no good fraudster. You just don't know.