The 'fiberboard' is probably Homasote, a cellulose product that really gained popularity in WWII, and shortly there after, still made and still acceptable in its modern form. Its basically a treated paper mache like material with paper backing. The anti-vermin treatment chemicals were originally some that would not be allowed today and may have "worn out."
As joe says the termites did not appear out of no where. They nest underground and come up the foundation thru earthen tubes to framing and then to paper backing, they will go for easiest thing to chew first. So infestation may be recent. Did exterminator go under house?
If still interested in buying house, do not rely on free estimates from exterminator, hire a truly independent termite inspector, unassociated with any pest control company, you current home insuarance company may be able to recommend an inspector. You may be able to hire inspector from exterminator if you make it clear that you will not be hiring them to do extermination, regardless of report.
If seller is aware of your interest, they should be willing to pay for removing some of Homasote and replacing it for inspection. If not I'd pass on this house.
Anyone contemplating buying or building home should watch original Mr. Blandings Builds His Dream House, several times. The main lesson learned is that being married to Myrna Loy makes anything bearable.
[CENTER][FONT=Comic Sans MS]Measure twice, cut once.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]