From what I know...
DLA comes in two parts IIRC. One part for mobility needs, the second part for care needs.
If she (you) takes out a car on the scheme, she will lose
all the DLA money paid for her Mobility needs. - Unless you decide to choose a car that is crap, like a Fiat Panda, or some other PoS, and they then don't take the whole amount of benefit, they will leave her with a few quid (literally) coming in.
Depending on vehicle choice, you may have to pay a deposit up front. (Which you don't get back.)
Can she survive financially if she loses the £xxx+ money coming in currently...? The mobility side is a big chunk of money in the DLA...
It's a decent scheme, but has it's pitfalls. The contract is notoriously tricky to get out of. Change of mind/circumstances won't cut it. Medical grounds only. (E.G. Disability greatly changed/worsened.)
The bigger/better the car, the more you pay. (Upfront usually)
Exceed the annual mileage and you'll get stung usually. (12000 miles a year) Garage sends all these details back to Motability during annual service, which must be carried out.
Car must be handed back in reasonable condition at the end of the three years. Any work that needs to be done will be carried under the insurance which you'll pay the excess for. (Sometimes multiple claims may have to be done, which means multiple excess fees as well...)
The car will definitely be taken back, and possibly DLA stopped if the fraud guys think you're taking the piss. And
if you are taking the piss, I hope they throw the book at you...!
No No-Claims-Bonus exist for the scheme. (So, if at the end of the three (etc) years, you decide to buy you own car again, you won't get any NCB's. IIRC, they might give you a letter to give to other insurers, but whether they accept that letter is up to them.)
But when all said and done, for the purpose it's intended for, it's a good enough scheme. Especially if only need a small car.
BTW, I'm only talking about Contract Hire, not the HP side of things. And my info may be a bit out of date by now, so don't take everything as gospel...