I second that. If the cabinet is wooden, an open back or cosmetic back made from cardboard can be replaced by a bit of plywood or MDF that has the mechanical strength to support a standard VESA wall mount without damaging the surfaced part of the cabinet. An articulating wall mount will allow the flat screen to be adjusted inside the cabinet to allow access to the back of the set and inside the cabinet. With a larger flat screen that can't fit entirely inside the cabinet, the same mount can hold the set in front of the cabinet.
Anyone with basic woodworking skills and a minimal set of home tools can get the materials in most big box home improvement stores. The structural backing plate can be cut to size at the store. Tools that may be needed to drill or cut holes in the panel to allow for cables and ventilation, and perhaps a nail or staple gun to use to attach it securely to the cabinet can be rented. If the job is done with care, the panel can be removed when it comes time to sell the cabinet, or use it for other things.
When it comes to metal furniture, that's a job best left to the pros. If the cabinet's owner has to ask, I'd presume that they don't possess the tools or the skills to DIY something like that. The only exception is with professional equipment cabinets with standard 19" rack mounts. There are a variety of professional rack mount monitor products available, albeit at professional prices.

How about adding a chunk of plywood the fits the opening tightly, attached with brackets or cleats to the cabinet shelves and sides? Then mount your choice of wall mount to that. Or, if you can weld, make something out of scrap steel. Or, visit an AV specialty store; they'll may have some mounts that'll work.

I have an entertainment center that goes back pretty deep (probably a good three-four feet). My 42" screen fits perfectly in the space.
Anyways, recently we purchase a new 55" TV from Best Buy and they came out and just wall mounted it. It stuck out just over the edge of the entertainment center.
Unfortunately we returned the TV because we had not realized that LED TVs wash out. We currently have a plasma.
If you have any questions feel free to ask. I think in a situation like this it may be better to have a professional come out and wall mount it because it seemed quite difficult (it took them a long time).